It's not often very useful to name tests simply by incrementing numbers.
This change adds a mechanism by which a name can be generated using both
the index of the type in the type list, and also some from the type
itself somehow.
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.