Support multiple reporters for `catch_discover_tests` about catch2

[horenmar]

If anyone wants to poke into this, note that to have multiple reporters active, they have to have different output destinations, or use the extended reporter specification to explicitly target the same output. Currently discover_tests only accepts one reporter and either lets it write to stdout, or it generates output filename for the tests based on the tests's name and prefix/suffix/folder arguments to the script.

For a general solution for N reporters, the script would have to accept "templated" reporter specs and understand how to rewrite them for the actual tests. For solution specific to this issue, there would likely need to be an extra argument along the lines of STDOUT_REPORTER, that would be handled specially to target stdout, and the default REPORTER argument would keep the current semantics.