iOS,macOS: Build/test test_metal_surface_unittests (flutter/engine#56586)

Years ago, @chinmaygarde created TestMetalSurface and wrote a unit test
for it in 2866095.

And some things that should not have been forgotten were lost. History
became legend. Legend became myth. And for four and a half years, the
test passed out of all knowledge. Until, when chance came, it ensnared a
new bearer. The test came to the creature @cbracken, who took it deep
into the tunnels of the GN build files. And there it consumed him. The
test brought to @cbracken unnatural long life. For 500 hours, it
poisoned his mind. And in the gloom of @cbracken's cave, it waited.
Darkness crept back into the forests of the world. Rumour grew of a
shadow in the East - whispers of a nameless fear. And the Test of Unit
perceived its time had now come.

Author: cbracken

engine/src/flutter/BUILD.gn

@@ -195,6 +195,7 @@ group("unittests") {
       "//flutter/shell/platform/embedder:embedder_a11y_unittests",
       "//flutter/shell/platform/embedder:embedder_proctable_unittests",
       "//flutter/shell/platform/embedder:embedder_unittests",
+      "//flutter/testing:testing_unittests",
       "//flutter/testing/dart",
       "//flutter/testing/smoke_test_failure",
       "//flutter/third_party/tonic/tests:tonic_unittests",

engine/src/flutter/testing/BUILD.gn

@@ -204,10 +204,8 @@ if (is_mac || is_ios) {
         "test_metal_surface_impl.h",
         "test_metal_surface_impl.mm",
       ]
-      deps = [
-        ":skia",
-        "//flutter/fml",
-      ]
+      deps = [ "//flutter/fml" ]
+      public_deps = [ ":skia" ]
 
       # Skia's Vulkan support is enabled for all platforms (except iOS), and so parts of
       # Skia's graphics context reference Vulkan symbols.
@@ -246,3 +244,24 @@ if (use_swiftshader) {
     ]
   }
 }
+
+if (enable_unittests) {
+  executable("testing_unittests") {
+    testonly = true
+
+    sources = []
+    deps = [
+      ":testing_fixtures",
+      "//flutter/third_party/googletest:gtest",
+      "//flutter/third_party/googletest:gtest_main",
+    ]
+
+    if (test_enable_metal) {
+      cflags_objc = flutter_cflags_objc_arc
+      cflags_objcc = flutter_cflags_objcc_arc
+
+      sources += [ "test_metal_surface_unittests.cc" ]
+      deps += [ ":metal" ]
+    }
+  }
+}

engine/src/flutter/testing/run_tests.py

@@ -431,6 +431,7 @@ def make_test(name, flags=None, extra_env=None):
       make_test('fml_unittests'),
       make_test('no_dart_plugin_registrant_unittests'),
       make_test('runtime_unittests'),
+      make_test('testing_unittests'),
       make_test('tonic_unittests'),
       # The image release unit test can take a while on slow machines.
       make_test('ui_unittests', flags=repeat_flags + ['--timeout=90']),