gpu_sysman_test.c: use real collectd functions in tests

gpu_sysman_test.c: use real collectd functions in tests

Drop most mocked functions and use real ones by linking libcommon.la.
This required mocking few other functions that it refers (but does not
link).

Support for a build including relevant C-files directly, e.g. for
whole program analysis, is added behind BUILD_STANDALONE define.

(It's also a fallback in case libcommon.la later changes to include
functionality that tester must override to be able to check correct
plugin functionality.)

Signed-off-by: Eero Tamminen <eero.t.tamminen@intel.com>