spinlock-pthread-adaptive-test failed on RHEL 8.0

Host: ec2-100-25-177-187.compute-1.amazonaws.com
Project: WiredTiger (develop)
Commit: diff: WT-6165 cursor operation hung on WT_PREPARE_CONFLICT (#5661)

• Don't loop forever on a prepare-conflict (an infinite loop will cause the entire run to
  eventually time out, but there's no point in waiting that long and the eventual timeout
  can obscure the real problem).

• Add an explicit error message to make it clear what went wrong.

• Add a comment to clarify time check. | 14 May 20 22:46 UTC
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