test: initial external unit test suite for ‘config.c’

This patch adds a separate (external) test binary with unit tests for
config.c

This is achieved by having the test’s source file include
config.c (yes, the C file, not the header file).

This allows us to call e.g. parse_section_*() directly, without having
to go through config_load() (which requires having a readable file at
“path”).

meson.build

@@ -272,6 +272,7 @@ endif
 
 subdir('completions')
 subdir('icons')
+subdir('tests')
 
 summary(
   {