RFR: 8367365: java/math/BigInteger/BigIntegerTest.java failed in jtreg timeout

[Raffaello Giulietti]

This PR
* Moves sqrt() and nthRoot() tests to a new subset.
* Removes a leftover `@run` tag without a timeout.
* Increases the explicit timeout from 400 to 480.
* Sorts the jtreg tags according to standard conventions.

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Commit messages:
 - 8367365: java/math/BigInteger/BigIntegerTest.java failed in jtreg timeout

Changes: https://git.openjdk.org/jdk/pull/27211/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27211&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367365
  Stats: 23 lines in 1 file changed: 10 ins; 9 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/27211.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27211/head:pull/27211

PR: https://git.openjdk.org/jdk/pull/27211