test: Set cwd to Project.getBase() as the default for ProjectTester

Set the default working directory for running tests to be the base of the project. Prior to this change, the working directory what what ever the current working directory happened to be. When running the `check` gradle task, this could easily be the workspace or some other project which can result in test failures if the test case expected the current directory to be the project's directory.

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>