GN: Build rules and fixes for (almost) all mojo unittest targets

mojo/public/c/system/tests/BUILD.gn

Index: mojo/public/c/system/tests/BUILD.gn
diff --git a/mojo/public/c/system/tests/BUILD.gn b/mojo/public/c/system/tests/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..579b88110b6ab069da2a361228af51e2bce6e677
--- /dev/null
+++ b/mojo/public/c/system/tests/BUILD.gn
@@ -0,0 +1,17 @@
+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+source_set("tests") {
+  visibility = [ "//mojo/public/cpp/system/tests:mojo_public_system_unittests" ]
+
+  deps = [
+    "//testing/gtest",
+  ]
+
+  sources = [
+    "core_unittest.cc",
+    "core_unittest_pure_c.c",
+    "macros_unittest.cc",
+  ]
+}