Minor cleanup in mfhdf test code

[derobins]

These tests have intermittent failures w/ NVidia's C compiler, so they are a high priority for cleanup.

• Initialize a buffer in test_valid_args() with sentinel values
• Add VERIFY_FLOAT/DOUBLE macros to squash float compare warnings
• Add F to float constants
• Fix warnings regarding mixing 32- and 16-bit integers
• Add a missing const value to a function signature