jamm on -javaagent instead of classpath

[apilloud]

This improves test coverage of the javaagent path, which is what is used on actual workers.

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Mention the appropriate issue in your description (for example: addresses #123), if applicable. This will automatically add a link to the pull request in the issue. If you would like the issue to automatically close on merging the pull request, comment fixes #<ISSUE NUMBER> instead.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

To check the build health, please visit https://github.com/apache/beam/blob/master/.test-infra/BUILD_STATUS.md

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI.

[lukecwik]

Note that this only changes the testing for JMH benchmarks which is fine if you want to update that.

Note that there is existing coverage that tests the docker images, here is a code pointer:

beam/sdks/java/core/src/test/java/org/apache/beam/sdk/SdkHarnessEnvironmentTest.java

Line 61 in 122ae34

public void testJammAgentAvailable() throws Exception {