EDK: Split the mojo_edk_system_unittests target.

mojo/edk/system/test/BUILD.gn

Index: mojo/edk/system/test/BUILD.gn
diff --git a/mojo/edk/system/test/BUILD.gn b/mojo/edk/system/test/BUILD.gn
index 313d7de1f3a99aa11143c68160e00b9a0ed633e7..220213762bdeb99b5a3e3e4b7147857bac537de0 100644
--- a/mojo/edk/system/test/BUILD.gn
+++ b/mojo/edk/system/test/BUILD.gn
@@ -91,10 +91,7 @@ mojo_edk_source_set("run_all_perftests") {
   ]
 }
 
-mojo_edk_source_set("unittests") {
-  testonly = true
-  mojo_edk_visibility = [ "mojo/edk/system:mojo_edk_system_unittests" ]
-
+mojo_edk_unittests("mojo_edk_system_test_unittests") {
   sources = [
     "random_unittest.cc",
   ]