RFR: 8294739: jdk/jshell/ToolShiftTabTest.java timed out

[Adam Sotona]

ToolShiftTabTest::testFixImport intermittently timeouts.
Reason is exhaustive busy waiting loop for completion  to be ready.
Proposed patch fixes the test loop.

Please review.

Thanks,
Adam

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Commit messages:
 - 8294739: jdk/jshell/ToolShiftTabTest.java timed out

Changes: https://git.openjdk.org/jdk/pull/10869/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10869&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294739
  Stats: 8 lines in 1 file changed: 3 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/10869.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10869/head:pull/10869

PR: https://git.openjdk.org/jdk/pull/10869