Make //third_party/abseil-cpp:test_support a source_set

Windows component debug compiles for V8 and ANGLE are failing due to
being unable to create a DLL for //third_party/abseil-cpp:test_support.
Make it a source_set instead of a component to fix the compile.

BUG=455179175

Change-Id: Id97bb6ea84d3184f583c3f19ef6ec23468103764
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7089834
Commit-Queue: Adam Rice <ricea@chromium.org>
Reviewed-by: Danil Chapovalov <danilchap@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1536599}
NOKEYCHECK=True
GitOrigin-RevId: dc70faa51004b814b54ffd347ce6af0da5f89216

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index f7d1789..2ba07e8 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -131,7 +131,7 @@
 }
 
 # Abseil testing utilities should only be included in test binaries.
-component("test_support") {
+source_set("test_support") {
   testonly = true
   public_deps = [ "//third_party/abseil-cpp/absl/hash:hash_testing" ]
   deps = [ ":absl" ]