With the merge of all the packages into llvm now we need to test
all those with llvm, but the tests are still scattered across different
test repos. This commit adds discover steps to gather all the tests.
ninja-build is distributed from extra repos such as epel.
Unfortunately this is not available in all supported archs
which makes it impossible to test with ninja-build.
My expectation is that in the future, more tests will land in this git,
and not all of them would be useful in gating time. AFAIK, CI systems do
run everything they can, therefore adding `when` to control when the
plan is executed - `is not defined` bit should allow anyone to run it
manually, while the rest of the condition should limit its presence to
gating events.
