Re-enable prebuilt_test

Use a temporary directory as the build directory during tests so files
don't get written to the source tree.

Also add a few more tests for prebuilts with no file specified.

Bug: 31800129
Test: m -j, make sure .soong.environment is not written to the source tree
Change-Id: I623bc114b2ff534c8df9fb3ce273e804711f8f05
diff --git a/Android.bp b/Android.bp
index 966c9b3..8821098 100644
--- a/Android.bp
+++ b/Android.bp
@@ -82,7 +82,7 @@
     ],
     testSrcs: [
         "android/paths_test.go",
-        //"android/prebuilt_test.go",
+        "android/prebuilt_test.go",
     ],
 }