[Flang] Fix failing plugin tests

The updated tests were originally added in
https://reviews.llvm.org/D109890 and are currently causing some
buildbots to fail.

This patch:
* adds missing items in the `REQUIRERS` list in tests
* adds `flangOmpReport` (the plugin library added in D109890) as a CMake
  dependency for tests (only when examples are enabled)

Differential Revision: https://reviews.llvm.org/D110682