RFR: 8302109: Trivial fixes to btree tests

[Coleen Phillimore]

Please review this trivial change.  One of the btree tests claimed to be single threaded but didn't pass -t1 so was the same as btree011.  Ran tier7 that runs these tests.

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Commit messages:
 - 8302109: Trivial fixes to btree tests

Changes: https://git.openjdk.org/jdk/pull/12490/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12490&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302109
  Stats: 9 lines in 4 files changed: 1 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/12490.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12490/head:pull/12490

PR: https://git.openjdk.org/jdk/pull/12490