With it being in META_MERGE, it's a sign that it's just there to be used by some other module that reads the meta files. The only module that I know of that uses test_requires is Module::Install, but I've never actually used it, so I don't know all of its intricacies (eg, if you need to hide stuff in META_MERGE)
Of course, if you dig through the Module::Install::API documentation, you'll find the following notes:
As of this writing, test_requires is just an alias for build_requires, but this may change in the future.
The values set by build_requires and test_requires are passed to ExtUtils::MakeMaker as a BUILD_REQUIRES attribute, which may fall back to PREREQ_PM if your ExtUtils::MakeMaker is not new enough.