test: move inspector test back to parallel, unmark flaky

After nodejs#21182, the test is
no longer flaky.

Since it doesn’t use a static port, moving it to parallel
seems justified.

Author: addaleax

test/sequential/test-inspector-port-zero-cluster.js renamed to test/parallel/test-inspector-port-zero-cluster.js

@@ -16,7 +16,6 @@ test-inspector-bindings                  : PASS, FLAKY
 test-inspector-debug-end                 : PASS, FLAKY
 
 [$system==linux]
-test-inspector-port-zero-cluster         :  PASS, FLAKY
 
 [$system==macos]