Add tests for ImfCRgbaFile.cpp

[cary-ilm]

Add tests to the test suite for the functions in src/lib/OpenEXR/ImfCRgbaFile.cpp.

These functions are C wrappers around the C++ API, so tests can follow the pattern of the other basic file I/O routines.

The tests should go in src/test/OpenEXRTest and follow the established pattern.