Separate system modules tests into their own file

java_test.go is getting too big.

Bug: 182402568
Test: m nothing
Change-Id: I9c3818f1d747a230fcb66687319a3a14d557a57c

java/Android.bp

@@ -78,6 +78,7 @@ bootstrap_go_package {
         "plugin_test.go",
         "rro_test.go",
         "sdk_test.go",
+        "system_modules_test.go",
     ],
     pluginFor: ["soong_build"],
 }