Add lots of testonly annotations to the GN build.

base/test/BUILD.gn

Index: base/test/BUILD.gn
diff --git a/base/test/BUILD.gn b/base/test/BUILD.gn
index e3312c2fb9925e1a51d84b8c4fce72a194b2505c..8af09252fd989a2ebea197f0e185bf66258db36c 100644
--- a/base/test/BUILD.gn
+++ b/base/test/BUILD.gn
@@ -9,6 +9,7 @@ if (is_android) {
 }
 
 source_set("test_support") {
+  testonly = true
   sources = [
     "expectations/expectation.cc",
     "expectations/expectation.h",
@@ -134,6 +135,7 @@ config("perf_test_config") {
 }
 
 source_set("test_support_perf") {
+  testonly = true
   sources = [
     "run_all_perftests.cc",
   ]
@@ -147,6 +149,7 @@ source_set("test_support_perf") {
 }
 
 source_set("run_all_unittests") {
+  testonly = true
   sources = [
     "run_all_unittests.cc",
   ]