Rename test file so it doesn't masquerade as a Picard interval list.

[cmnbroad]

When parsing interval arguments in GATK using the latest htsjdk, files that end in ".interval_list" are claimed by the new IntervalListCodec introduced in samtools/htsjdk#1327. This PR renames the one test file in GATK that has a Picard interval list file extension but isn't really a Picard interval list; without this change, CountReads and CountReadsSpark integration tests will fail when we upgrade to the next htsjdk.

[droazen]

👍 merge when ready @cmnbroad

[codecov-io]

Codecov Report

Merging #5879 into master will not change coverage.
The diff coverage is 100%.

@@             Coverage Diff             @@
##              master     #5879   +/-   ##
===========================================
  Coverage     86.833%   86.833%           
- Complexity     32292     32293    +1     
===========================================
  Files           1990      1990           
  Lines         149107    149107           
  Branches       16477     16477           
===========================================
  Hits          129474    129474           
  Misses         13621     13621           
  Partials        6012      6012

Impacted Files	Coverage Δ	Complexity Δ
...te/hellbender/tools/CountReadsIntegrationTest.java	91.837% <100%> (ø)	11 <0> (ø)	⬇️
...park/pipelines/CountReadsSparkIntegrationTest.java	98.246% <100%> (ø)	9 <0> (ø)	⬇️
...lotypecaller/readthreading/ReadThreadingGraph.java	88.725% <0%> (-0.245%)	159% <0%> (ø)
...nder/utils/runtime/StreamingProcessController.java	67.773% <0%> (+0.474%)	33% <0%> (ø)	⬇️