Add TestRequires

[cgwalters]

It's really common for software to have separate requirements to build versus actually run - and that distinction today exists in BuildRequires vs Requires.

Except...at build time in RPM today, the %check typically invokes at least unit tests of the software, and it's not uncommon at all for unit test suites to pull in some or all of the runtime requirements.

There's no way to express the difference; these have to be BuildRequires today - and for bootc for example I added BuildRequires: skopeo.

But there are valid use cases for building RPMs without running the test suite such as architecture bootstrapping.

I think it'd be cleaner if we had CheckRequires for example (or TestRequires) that are defined to be present when running %check (and for clean root builds, not when running %build).

[pmatilai]

See #2631