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>

scripts/dawn_overrides_with_defaults.gni

@@ -24,6 +24,10 @@ if (!defined(dawn_standalone)) {
   dawn_standalone = false
 }
 
+if (!defined(dawn_root)) {
+  dawn_root = get_path_info("..", "abspath")
+}
+
 if (!defined(dawn_jinja2_dir)) {
   dawn_jinja2_dir = "//third_party/jinja2"
 }