test: move inspector test back to parallel, unmark flaky
After https://github.com/nodejs/node/pull/21182, the test is no longer flaky.
Since it doesn’t use a static port, moving it to parallel seems justified. (No issue with running it in parallel locally.)
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines