Motivation:

It's annoying to have to wait for a long build to complete.
But arguably even worse: we have a limited number of build agents, and
longer builds increases their utilization and reduces their
responsiveness to new PRs, in turn adding a lengthy delay before builds
for new PRs even start.

Modification:

- Disable the `buddyAllocationConsistency` test when the leak detector
is enabled. This test took up to seven minutes to complete when the
paranoid leak detector was enabled - and we ran it twice!
- Make the buffer tests run in parallel. Locally this cuts the build
time for the buffer module in half.
- Isolate the JFR event tests because they're inherently inspecting
static mutable state.
- Make the handler tests run in parallel per class. This allows the
many, fairly slow SSL engine tests to run in parallel.

Result:

Much faster build - especially when the paranoid leak detector is
enabled.

(cherry picked from commit f0fe411)