Tests: Avoid duplicate rules in ExternalData test
In commit v3.7.0-rc1~158^2 (ExternalData: Tolerate files duplicated across multiple targets, 2016-07-07) 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.
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