Run testdmabuf only on Linux

This test includes a Linux-specific header and therefore breaks on
non-Linux OS.

This change fixes building the tests on macOS and therefore enables
that in CI by not disabling them anymore.

.gitlab-ci.yml

@@ -251,7 +251,6 @@ macos-x86_64:
             -Dpixman:tests=disabled
             -Dlibjpeg-turbo:simd=disabled
             -Dbuild-demos=false
-            -Dbuild-tests=false
             -Dbuild-examples=false
             -Dbuild-testsuite=false
             _build

tests/meson.build

@@ -127,6 +127,11 @@ gtk_tests = [
 if os_unix
   gtk_tests += [
     ['testfontchooserdialog'],
+  ]
+endif
+
+if os_linux
+  gtk_tests += [
     ['testdmabuf'],
   ]
 endif