Add integration tests for the C backend #544

Currently we only test the --show-c with the ./run_tests.py testsuite, which checks the generated C output. We also have to add integration tests that will test:

• actually compiling the C code via a C compiler
• linking multiple files
• testing the C interop (call into C, callback from C, etc.)
• testing importing C header files (Import from C Header File #541)

We probably can extend our current test framework, to test llvm and/or C with the same test files. We have to add the ability to specify other C files to compile and link.

I think the frontend should be improved to be able to use syntax like:

lpython --backend=c -c some_file.py -o some_file.c

Then we compile everything with gcc or clang.