tests: fix integration test file handling

The integration tests were failing with messages about build.gradle already existing.
This appears to be caused by the tests making the assumption that TemporaryFolder.newFolder/newFile
would return the same file if called repeatedly with the same arguments.  However, it looks like that was
changed such that now it throws an exception if the file exists.

The new implementation should meet the expectations of the existing tests.

https://github.com/junit-team/junit/issues/413
https://github.com/junit-team/junit/pull/419
configure idea module scopes (fixes #24)

This makes it so that libraries that are on the grails classpath end up in the IntelliJ module file,
allowing IntelliSense to work as intended.  Other usages (such as run configurations) may not work yet.
Merge pull request #28 from davidmc24/issue-24

Expose Grails dependencies to Idea plugin (fixes #24)