Create new src/tests BUILD.gn file.

Move all test-related build stuff into its own BUILD.gn file. This
required moving the dawn_generator template into a common file, so it
can be called by both BUILD.gn and src/tests/BUILD.gn.

[This is a reland of https://dawn-review.googlesource.com/c/dawn/+/2940
with a fix for mock_dawn.]

Bug: dawn:61
Change-Id: Id1e6d0c2b07caa2610cebe206511e972ac18fe8d
Reviewed-on: https://dawn-review.googlesource.com/c/3020
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Stephen White <senorblanco@chromium.org>

generator/templates/mock_api.cpp

@@ -12,7 +12,7 @@
 //* See the License for the specific language governing permissions and
 //* limitations under the License.
 
-#include "mock_dawn.h"
+#include "mock_dawn_.h"
 
 namespace {
     {% for type in by_category["object"] %}