b77cec05c0 fixed this problem for tagging tests, so that a test
case tagged with `[.foo]` would be parsed as tagged with `[.][foo]`.
This does the same for the test spec parsing.

Fixes #1798

The JUnit report is improved in that:
* The message shows the testing condition, not the result
* The actual message has similar output than the console one

It outputs reports in the `Generic Execution Test Data` format, see
https://docs.sonarqube.org/latest/analysis/generic-test/, specifically
https://docs.sonarqube.org/latest/analysis/generic-test/#header-2

Close #1738 (this is a cherry-pick and fixup of that PR)

Now it no longer tries to be this weird hybrid between an owning
and non-owning reference, and is only ever non-owning. This is also
reflected in its interface, for example `StringRef::isNullTerminated`
is now public, and `StringRef::c_str()` has the precondition that it
is true.

Overview of the changes:
* The `StringRef::m_data` member has been completely removed, as it
had no more uses.
* `StringRef::isSubstring()` has been made public and renamed to
`StringRef::isNullTerminated()`, so that the name reflects what the
method actually does.
* `StringRef::currentData()` has been renamed to `StringRef::data()`,
to be in line with common C++ containers and container-alikes.
* `StringRef::c_str()` will no longer silently make copies. It instead
has a precondition that `isNullTerminated()` is true.
* If the user needs a null-terminated string, they should use the
`std::string` conversion operator and call `c_str()` on the resulting
`std::string`.
* Some small optimizations in various places.
* Basic functionality is now `constexpr`.

This should now properly handle small numbers which would previously
output something like `[0.00000000000000019, 0.00000000000000019]`,
which does not allow user to read the numbers properly.

Closes #1760

Only works for exceptions that publicly derive from `std::exception`
and the matching is done exactly, including case and whitespace.

Closes #1649
Closes #1728

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It checks Knuth's _close enough with tolerance_ relationship, that
is `|lhs - rhs| <= epsilon * max(|lhs|, |rhs|)`, rather then the
_very close with tolerance_ relationship that can be written down as
`|lhs - rhs| <= epsilon * min(|lhs|, |rhs|)`.

This is because it is the more common model around the internet, and
as such is likely to be less surprising to the users. In the future
we might want to provide the other model as well.

Closes #1746