util: Add tests for the string buffer

util: Add tests for the string buffer

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