gio2 test: add testutils for skip test for OS X
gio2 test: require GioTestUtils::Mac in gio-test-utils
gio2 test: skip tests for ContentType guess on OS X

these tests doesn't work on OS X
gio2 test: add underscore

omit_on_osx ->
omit_on_os_x
          ^
gio2 test: rename module name

GioTestUtils::Mac ->
GioTestUtils::Omissions