test: move debugger tests to sequential

The debugger tests in parallel fail with `make test` sometimes (all the
time?). This appears to be related to running in parallel, as it does
not fail with `make test-ci`, when run via `tools/test.py` or directly
from the command line with `./node
test/parallel/test-debugger-util-regression.js`.

A separate issue may be opened to find out why it is failing in
parallel, but for now, I think it's important to fix `make test`
promptly.

I suspect the issue is that the tests are relying on a default port
somewhere and so they are colliding when run in parallel. But that's
just a guess for the moment.

Fixes: nodejs#6201

Author: Trott