gn: Add 'test_only' label to util targets.

Clarifies that angle_util is only used in tests and samples.

Bug: angleproject:3162
Change-Id: I9a578cf7a9a09b2c3fd4683155824ed2e72f68cc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1964934
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 0c4daf8..253e091 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1041,6 +1041,7 @@
 }
 
 angle_source_set("angle_util_loader_headers") {
+  testonly = true
   sources = [
     "util/egl_loader_autogen.h",
     "util/gles_loader_autogen.h",
@@ -1057,6 +1058,7 @@
 }
 
 angle_source_set("angle_util_loader") {
+  testonly = true
   defines = [ "LIBANGLE_UTIL_IMPLEMENTATION" ]
 
   sources = [
@@ -1102,6 +1104,7 @@
   }
 
   target(library_type, library_name) {
+    testonly = true
     sources = util_sources
     deps = [
       ":angle_common",

samples/BUILD.gn

diff --git a/samples/BUILD.gn b/samples/BUILD.gn
index 020b05f..3887ac3 100644
--- a/samples/BUILD.gn
+++ b/samples/BUILD.gn
@@ -17,6 +17,7 @@
 }
 
 angle_static_library("sample_util") {
+  testonly = true
   sources = [
     "sample_util/SampleApplication.cpp",
     "sample_util/SampleApplication.h",
@@ -53,6 +54,7 @@
   }
 
   angle_executable(target_name) {
+    testonly = true
     forward_variables_from(invoker,
                            [
                              "cflags",

src/tests/BUILD.gn

diff --git a/src/tests/BUILD.gn b/src/tests/BUILD.gn
index a2ab137..cb14eb7 100644
--- a/src/tests/BUILD.gn
+++ b/src/tests/BUILD.gn
@@ -591,6 +591,7 @@
 
   template("angle_deqp_source_set") {
     angle_source_set(target_name) {
+      testonly = true
       forward_variables_from(invoker, "*")
       suppressed_configs += deqp_undefine_configs
 
@@ -808,6 +809,8 @@
                                "data_deps",
                              ])
 
+      testonly = true
+
       deps += [
         ":angle_deqp_libtester",
         "$angle_root:angle_util",
@@ -826,6 +829,7 @@
 
     if (!build_with_chromium) {
       angle_executable("angle_deqp_${_api}_no_gtest") {
+        testonly = true
         sources = [
           "deqp_support/angle_deqp_tests_main.cpp",
         ]