tests: build the test icon theme for all platforms.

In particular, we were not building these for win32, which has no
symbolic link concept. So let's use $(LN_S) defined by autoconf for this
purpose, as it will copy files for platforms without ln support.
Moreover this way, we don't create one such folder for each and every
test run and fill up the tmp directory with countless directories, that
we never clean up.
On Win32, this fixes 3 tests which were failing because of the missing
icons.
tests: gimp_test_utils_synthesize_key_event() not working for win32.

gdk_test_simulate_key() has currently no win32 implementation.
Add this implementation in our code until a patch to GTK+ for this is
merged. This fixes 2 `make check` tests for win32.
tests: make warnings non fatal for tests on win32.

g_test_init() makes any warning message fatale by default. While it
makes sense for the main OS that we build on (Linux), I believe that
our tests on Windows are more useful when they finish, even with some
warnings, than stopping because some stupid font warning would occur
(case I encountered, did not know the solution of, and is not really
relevant to the rest of the test).