[flang] Extend test_errors.py to test warnings and explanatory messages

flang/test/Semantics/test_errors,py only compares actual error messages
with expected error messages.  Many tests have expected warning messages
in them, but they are not checked.

A forthcoming change adds several new warning and explanatory messages
to the compiler, and these messages must be testable.

So (re-?) enable non-error message checking in test_errors.py and adjust some
existing tests to get them to pass.

Warning messages related to host-specific folding conditions will not
be emitted on all platforms, so they will continue to be ignored.

Differential Revision: https://reviews.llvm.org/D136479