test: remove watchdog in test-debug-signal-cluster

[Trott]

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test debugger

Description of change

test-debug-signal-cluster contains a watchdog timer that results in
false positives in CI. Remove the watchdog timer and let the test runner
determine that the test has timed out.

Example failure: https://ci.nodejs.org/job/node-test-commit-smartos/5075/nodes=smartos14-64/console

not ok 217 parallel/test-debug-signal-cluster
# got pids [81560,84923,85367]
# 
# assert.js:85
#   throw new assert.AssertionError({
#   ^
# AssertionError: test timed out
#     at Object.exports.fail (/home/iojs/build/workspace/node-test-commit-smartos/nodes/smartos14-64/test/common.js:443:10)
#     at Timeout.testTimedOut [as _onTimeout] (/home/iojs/build/workspace/node-test-commit-smartos/nodes/smartos14-64/test/parallel/test-debug-signal-cluster.js:69:10)
#     at ontimeout (timers.js:365:14)
#     at Timer.unrefdHandle (timers.js:466:3)
# > all workers are running
# > Starting debugger agent.
# > Debugger listening on 127.0.0.1:12646
# > Starting debugger agent.
  ---
  duration_ms: 7.124

CI stress test on current master showing failures: https://ci.nodejs.org/job/node-stress-single-test/1053/nodes=smartos14-64/console

CI stress test on this change (will hopefully come up green): https://ci.nodejs.org/job/node-stress-single-test/1054/nodes=smartos14-64/console

[Trott]

CI: https://ci.nodejs.org/job/node-test-pull-request/4796/

[Trott]

Landed in b0486b1