Revert "Create new src/tests BUILD.gn file."

This reverts commit 672d29d14c.

Reason for revert: https://bugs.chromium.org/p/chromium/issues/detail?id=913171

Original change's description:
> 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>

TBR=cwallez@chromium.org,kainino@chromium.org,senorblanco@chromium.org

Change-Id: I54cdc558b128935dc8a8d22ec2b5e879271c35ae
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: dawn:61
Reviewed-on: https://dawn-review.googlesource.com/c/3080
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

scripts/dawn_overrides_with_defaults.gni

@@ -24,10 +24,6 @@ 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"
 }