Catch2 produces tests names with spaces by default when using
type-parametric macros like `TEMPLATE_TEST_CASE` and
`TEMPLATE_PRODUCT_TEST_CASE`. For some test-runners that are
incompatible with spaces in test names, this presents a significant
problem: It's possible to define a test that's impossible to run.
This adds a `CATCH_CONFIG_NAME_SEPARATOR` configuration value that can
be used to override the separator string used during test naming.
