[lit] Add a method to lit.TestFormat to get the list of tests associated to a path

Lit TestFormat classes already needed to implement the getTestsInDirectory
method. However, some test formats may want to expand a single test path
to multiple Lit tests, for example in the case of a test that actually
generates other Lit tests.

To accommodate that, this commit adds the getTestsForPath method to
TestFormat. This method can be used to turn a single path in a Lit
test suite into a list of tests associated to that path.

Differential Revision: https://reviews.llvm.org/D151664