Tests: Avoid duplicate rules in ExternalData test

In commit f9973166 we added a test case for repeating the same data file reference in three different targets in the same directory in order to exercise its corresponding fix. However, we re-used the top-level Data.dat file which is already produced by an unrelated test case. ninja diagnoses the duplicate rule.

Update the test to use a dedicated data file name for the three-times repeated case to avoid this conflict. It still covers the original purpose of the test.