RFR: 8303773: Replace "main.wrapper" with "test.thread.factory" property in test code

[Leonid Mesnik]

The main.wrapper was the first name for jtreg test thread factory plugin. However, during integration of this feature in jtreg it was decided to use test.thread.factory name. So this fix just renames "main.wrapper" property to  "test.thread.factory" so it is more compliant with jtreg naming. Also, it makes more sense for tests when it is used to create other then main threads in test.
Testing: tier1-5.
Verified that "main.wrapper" is not used in test sources anymore.

I haven't rename DebugeeWrapperd and MainWrapper classes in JDI test frameworks because they are actually more main wrappers than thread factories.

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Commit messages:
 - fixed tests

Changes: https://git.openjdk.org/jdk/pull/15950/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15950&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303773
  Stats: 63 lines in 21 files changed: 0 ins; 1 del; 62 mod
  Patch: https://git.openjdk.org/jdk/pull/15950.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15950/head:pull/15950

PR: https://git.openjdk.org/jdk/pull/15950