GN: chromeos: Add missing components tests

BUG=504084

Review-Url: https://codereview.chromium.org/2104203003
Cr-Original-Commit-Position: refs/heads/master@{#403202}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f854323abcf7453e80982fee451eafb1cc200ed8
diff --git a/BUILD.gn b/BUILD.gn
index d6a7efb..c6f4a12 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -12,3 +12,17 @@
     "//base",
   ]
 }
+
+source_set("unit_tests") {
+  testonly = true
+
+  sources = [
+    "alarm_timer_unittest.cc",
+  ]
+
+  deps = [
+    ":timers",
+    "//base",
+    "//testing/gtest",
+  ]
+}