Rename C API tests so they have .cpp extension. This is necessary
because GoogleTest is a C++ library and so the test must be built
with a C++ compiler.
Enabled most C api tests. Most of them are deliberatly broken because
they are terrible tests and don't have any assertions!

A few tests have been missed out because they are leaks (I'm not sure
how to programmatically test for memory leaks) or I don't know how to
run the program.