Catch2 defaults to random test execution order, which has to process
all tests present in the binary, even if they will be filtered down
to just 1 later. For the `SelfTest` binary, the overhead from this is
about 20% in Debug configuration and 5% in Release.

By registering the CTest tests with `--order decl`, we avoid this
overhead.

Closes #2149

There were 2 separate bugs, one recently added and one ancient
(since roughly the first version of the registration script).

1) The recent refactoring of how the JSON output from Catch2 is
   parsed caused shadowing between the variable containing the
   per-test JSON fragments and the accumulator of test names for
   <target>_TESTS variable. This led to the variable containing
   both the names and the JSON fragments, and thus being completely
   wrong.

2) The test name accumulation has never accounted for characters
   that need escaping to be present in a CMake list. This means
   that e.g. test names with semicolon in them would end up with
   two elements in the test list.

Both of these are now fixed, at the cost of extra complexity and
my sanity as I had to learn more about CMake escaping rules.

(CMake escaping rules are dumb)

This allows it to deal with badly behaved code, where 3rd party
dependencies write into stdout during global construction.

Closes #3162
Closes #3166

We also bump the minimum CMake version to 3.20 as per #2943