Always build osmesa (on Linux) and run ui_unittests.

(The only (?) dep to osmesa was removed in
28dd561326b6d885e105e6bfa34f9b2770ce3910.)

Also, make osmesa test-only.

R=jamesr@chromium.org, jeffbrown@google.com

Review URL: https://codereview.chromium.org/1649993002 .
diff --git a/BUILD.gn b/BUILD.gn
index de6c256..18f558f 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -21,7 +21,12 @@
   ]
 
   if (is_linux) {
-    deps += [ "//fusl" ]
+    deps += [
+      "//fusl",
+
+      # We need this to run tests (especially on bots).
+      "//third_party/mesa:osmesa",
+    ]
   }
 
   # TODO(cstout): fix sandbox build for fnl/musl