[flang] expand the this_image test coverage

Add a test with a range of this_image() intrinsic function
invocations, including a comprehensive set of standard-conforming
keyword and non-keyword arguments with and without optional
arguments present and with argument positions covering all
possible orderings.  Also test that several non-conforming
this_image() invocations generate the correct error messages.

Differential Revision: https://reviews.llvm.org/D123331