More tests could probably be added, but this should cover
concatenation, resizing, clearing, formatted printing,
and checking the length, so it should be quite complete.

Signed-off-by: Thomas Helland <thomashelland90@gmail.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle at amd.com>
Tested-by: Dieter Nützel <Dieter at nuetzel-hh.de>

V2: Address review feedback from Timothy, plus fixes
   - Use a large enough char array
   - Actually test the formatted appending
   - Test that clear function resets string length

V3: Port to gtest

V4: Fix test makefile
    Fix copyright header
    Fix missing extern C
    Use more appropriate name for C-file
    Add tests for append_char