Fix a test case failure in the C# grade.

Avoid some warnings during runtests.

tests/hard_coded/write.exp3:
	Update this expected output.

tests/hard_coded/hash_bug.m:
	Do not call an obsolete function.

tests/hard_coded/Mercury.options:
	Do not warn about unresolved polymorphism in a test.
Cleanups for tests/hard_coded.

tests/hard_coded/Mmakefile:
	Only run the target_mlobjs test case in C grades; directly linking
	against an object file isn't possible in the non-C grades.

tests/hard_coded/Mercury.options:
	Suppress irrelevant warnings generated by some tests.

tests/hard_coded/hash_init_bug.m:
	Replace a call to an obsolete function.
Fix the failure of tests/hard_coded/bigtest in the C# grade.

tests/hard_coded/lp.m:
	Add a C# foreign_proc for mkuniq/2.