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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