Those screenshot cppunit tests need to run in non-headless mode with
dialog windows exposed (i.e. VCL_HIDE_WINDOWS must not be set)

To preserve the current behaviour (VCL_HIDE_WINDOWS is set always on Win
and Mac, and in use_vcl_non_headless case on Linux), introduce 2 new
variables:

gb_CppunitTest_show_windows: unset VCL_HIDE_WINDOWS unconditionally
gb_CppunitTest_hide_windows: set VCL_HIDE_WINDOWS=1 if HEADLESS is empty

(i.e. let's not use emptiness of HEADLESS as an indicator, as it is
empty in 2 mutually exclusive cases)

Change-Id: Ib2f7a9cfb1202944d10856c44b6ac7c41156b333