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”).

tests/meson.build

@@ -0,0 +1,8 @@
+config_test = executable(
+  'test-config',
+  'test-config.c', '../tokenize.c',
+  wl_proto_headers,
+  link_with: [common],
+  dependencies: [pixman, xkb, fcft])
+
+test('config', config_test)