RFR: 8294137: Review running times of java.math tests

[Brian Burkhalter]

Break up `BigIntegerTest` and `SymmetricRangeTests` into subsets which are run quasi-randomly as a function of second within the minute at the time the test is running. `LargeValueExceptions` does not appear amenable to this treatment.

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Commit messages:
 - 8294137: Review running times of java.math tests

Changes: https://git.openjdk.org/jdk/pull/12819/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12819&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294137
  Stats: 126 lines in 3 files changed: 50 ins; 5 del; 71 mod
  Patch: https://git.openjdk.org/jdk/pull/12819.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12819/head:pull/12819

PR: https://git.openjdk.org/jdk/pull/12819