Install intermediate file in testcases

ctx.InstallFile adds the source file to checkBuildFiles. This causes
issues if the module is an unexported namespace, since the installation
files will not be created.

Additional context: https://android-review.git.corp.google.com/c/platform/build/soong/+/3474792/comment/40af1b92_5e65d152/

Test: m nothing --soong-only
Test: go test ./java
Change-Id: I83ac66c6a81c8163a24acd4e9ef8aab0982e5e4c