Fix intermittent failure of TestDoubleValuesSourceRescorer

[ChrisHegarty]

This commit fixes an intermittent failure of TestDoubleValuesSourceRescorer, where the test sometimes randomly selects too few documents to index leading to no matches in the search. Failure example:

org.apache.lucene.search.TestDoubleValuesSourceRescorer > test suite's output saved to /../lucene/core/build/test-results/test/outputs/OUTPUT-org.apache.lucene.search.TestDoubleValuesSourceRescorer.txt, copied below:
   >     java.lang.ArrayIndexOutOfBoundsException: Index 0 out of bounds for length 0
   >         at __randomizedtesting.SeedInfo.seed([86E2E915B8727B0:A3943384845BA19E]:0)
   >         at org.apache.lucene.search.TestDoubleValuesSourceRescorer.testBasic(TestDoubleValuesSourceRescorer.java:99)
   >         at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
   >         at java.base/java.lang.reflect.Method.invoke(Method.java:580)
...

I think that bumping the number of docs to index should be sufficient, as I see no failures even after several hundreds of thousands runs.

relates #14776

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[vigyasharma]

Oops, Thanks for fixing this Chris!