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

This reverts commit 0a7ddd43d7.

Reason for revert: Broke compile in https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/linux-rel/12314
See also: https://chromium-review.googlesource.com/c/chromium/src/+/1364694

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.
> 
> Bug: dawn:61
> 
> Change-Id: Icaa459270bdaa60306e053b93835812e70dba6f5
> Reviewed-on: https://dawn-review.googlesource.com/c/2940
> Reviewed-by: Kai Ninomiya <kainino@chromium.org>
> Reviewed-by: Stephen White <senorblanco@chromium.org>

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

Change-Id: If6d8c759f63c12b244471016f10e92c07299303f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: dawn:61
Reviewed-on: https://dawn-review.googlesource.com/c/3000
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@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"
 }