[flang] Add executable tests for conditional expressions (F2023)

[cenewcombe]

I ported over the tests we had in tst90 for conditional expressions. The CMakeLists.txt file excludes tests with conditional arguments for now. This can be altered once the feature is implemented with support for .NIL.

Let me know if this follows the correct format for porting from tst90.

I'll keep this PR in my fork until the conditional expressions feature work is ready to go upstream.

[tmjbios]

Do you have a test that checks for an errant evaluation of a non-used consequent?

[cenewcombe]

Do you have a test that checks for an errant evaluation of a non-used consequent?

Yep, we have

condexpr_scalar_call_01.f90 - tests functions only called for the selected branch
condexpr_scalar_call_02.f90 - errant evaluation causes div by 0

condexpr_nil_05.f90 checks selective evaluation for conditional arguments, and is currently disabled pending implementation of that feature.

[cenewcombe]

I've updated two of the conditional arguments tests to remove parentheses for chained conditional-arg:

condexpr_nil_02.f90
condexpr_nil_03.f90

This matches my understanding of the discussion regarding conditional arguments, but please let me know if I've misunderstood. These tests are not currently enabled, but I want to port them as we'd expect to run them.

I've left the conditional expressions tests as-is since extra parentheses should be OK in that case.
cc: @vntkmr @tmjbios