Catch3 keeps Catch2's API for writing tests (test cases, sections,
assertions, matchers, generators, listeners, benchmarks) and replaces its
session, command line and reporters with a runner of its own.

The runner starts the test binary again as worker processes that take the
next test when they are free. A crash, an exit or a hang ends one test, not
the run. On top of that it adds:

- retries that tell deterministic, flaky and order-dependent failures
  apart, and a bisection that names the test leaving state behind;
- a history of verdicts and durations, kept under a file lock, that drives
  automatic quarantine, adaptive time limits, predictive ordering,
  incremental runs, failure replay and analytics;
- selection by change, past failure or prediction, dependency closure,
  and sharding by a stable hash or by recorded durations;
- tags for serial and isolated tests, shared resources, dependencies,
  time limits and retries;
- a failure category, a signature, an analysis and a replay command for
  every failure;
- console, JUnit XML (with flakyFailure and rerunFailure), JSON and Chrome
  trace reports, and a versioned discovery listing (--list=json);
- a temporary directory per test (TMPDIR, TMP, TEMP and
  TestTemporaryDirectory());
- Bazel's test environment: XML output file, test filter, sharding, seed,
  premature exit file and undeclared outputs.

The command line is parsed by a hand-written, typed parser. Catch::Session
remains as a thin facade over the runner for custom main functions.

The result model in Progmasoft/Catch3/Results.hpp was rewritten around
tests and attempts, and the JUnit writer now writes it. The property
engine's example database is updated under a file lock with an atomic
rename, so parallel workers can write it at the same time.

Removed: Clara, the command-line and reporter-spec parsers, every reporter
and CATCH_REGISTER_REPORTER, listing, text flow, console colour and width,
the JSON and XML writers, sharding in the test registry, approval tests and
their baselines, Conan packaging, and the unused third_party/clara.hpp.
Listeners keep working; reporters' list and skip events are gone.

The Catch2 library is now compiled as C++20; code that includes only
Catch2's headers still needs C++14. Duplicate test cases are still refused
at start-up. Version 0.3.0.

ExtraTests now turn exceptions off for clang-cl with MSVC-style options,
which -fno-exceptions did not do.

This provides nice speedup when writing strings that don't need
escaping, at about 6% in Debug build and ~40% in Release build.
If the strings do need escaping the speedup is much smaller, at
~1% in Debug and ~6% in Release build, as the cost of actually
escaping the strings dwarves the cost of checking.

In practice, the majority of values the JSON writer handles are
strings, but this makes the code more obvious and can still provide
a small speed-up.

As the types and values sent into the writer are determined by Catch2,
I do not expect to actually need this support, but it is better to have
it and not be surprised in the future.

Closes #3176

The old code was exceedingly simple, as it went char-by-char and
decided whether to write it to the output stream as-is, or escaped.
This caused a _lot_ of stream writes of individual characters.

The new code instead looks for characters that need escaping, and
bulk-writes the non-escaped characters in between them. This leads
to about the same performance for strings that comprise of only
escaped characters, and 3-10x improvement for strings without any
escaping needed.

In practice, we should expect the former rather than the latter,
but this is still nice improvement.

* bugprone-branch-clone
* bugprone-copy-constructor-init
* bugprone-empty-catch
* bugprone-sizeof-expression
* bugprone-switch-missing-default-case
* bugprone-unused-local-non-trivial-variable
* clang-analyzer-core.uninitialized.Assign
* clang-analyzer-cplusplus.Move
* clang-analyzer-optin.cplusplus.VirtualCall
* modernize-loop-convert
* modernize-raw-string-literal
* modernize-use-equals-default
* modernize-use-override
* modernize-use-using
* performance-avoid-endl
* performance-inefficient-string-concatenation
* performance-inefficient-vector-operation
* performance-noexcept-move-constructor
* performance-unnecessary-value-param (and improve generator example)
* readability-duplicate-include
* readability-inconsistent-declaration-parameter-name
* readability-non-const-parameter
* readability-redundant-casting
* readability-redundant-member-init
* readability-redundant-smartptr-get
* readability-static-accessed-through-instance
* unused variable in amalgamted tests

Together with liberal use of `_sr` UDL to compile-time convert
string literals into StringRefs, this will reduce the number of
allocation and remove most of the strcpy calls inherent in
converting string lits into `std::string`s.

Co-authored-by: Martin Hořeňovský <martin.horenovsky@gmail.com>