[crac] RFR: 8377177: [CRaC] Clarify engine use in the testsuite

[Radim Vansa]

If possible, test should use simengine.
If the test is specific to engine behaviour, it should use CracBuilder.engine(...) explicitly.
If the test can cover more generic behaviour of real engines, either use the default or accept the engine as test arg.

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Commit messages:
 - Imports cleanup
 - 8377177: [CRaC] Clarify engine use in the testsuite

Changes: https://git.openjdk.org/crac/pull/291/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=291&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377177
  Stats: 202 lines in 30 files changed: 90 ins; 78 del; 34 mod
  Patch: https://git.openjdk.org/crac/pull/291.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/291/head:pull/291

PR: https://git.openjdk.org/crac/pull/291