Previously, the `catch_discover_tests` would prepare the entire CTest
command (e.g. `add_test(...)` or `set_tests_properties(...)`) first, and
then escape it when finished. However, this caused lot of the command args
to be escaped over and over again (e.g. executable name or Catch2's reporter
args), for no reason, as they were always the same, and thus their escaping
was always the same.

Until recently, the performance overhead didn't matter as there were many
spots which had quadratic runtime in number of tests. However, the recent
refactorings fixed these, and this commit now improves the throughput by
10-20%.

Also extended the benchmarked COUNTS in `benchmark_discovery.py`,
because the performance is now good enough that it is reasonable
to benchmark 16k tests.

The previous approach was for `add_command` to behave as append function
via concatenating the command string internally and then saving it into
`PARENT_SCOPE`. Because this in practice ended up meaning concatenating
a copy of the string inside the function and then overwriting the string
outside the function, the performance was lacking.

The new approach is for `prepare_command` to only escape & return the
command from single call, and the caller is responsible for concatenating
the result. Since the actual concatenation no longer crosses function
scope boundaries, the performance is much better, even though the scaling
is still quadratic.

The new approach only takes 1/4-1/5 of the time, saving ~1s at 1k tests,
4.5s at 2k tests and 19s at 4k tests.