child_process: make fork() execPath configurable

Allows for arbitrary path to executable spawned using `fork`. This
fixes some issues around running multiple versions of node with workers
and allows arbitrary IPC with compatible executables.

Fixes #3248.
test: disable simple/test-debug-brk-file

This test is timing sensitive and hence quite unreliable with debug
builds. What's worse is that it leaves a stray child process behind
that listens on the default test port and that makes all the tests
that come after it fail with EADDRINUSE errors.
test: put tty in blocking mode after test

Tests can leave the tty in non-blocking mode. If the test runner tries
to print to stdout/stderr after that and the tty buffer is full, it'll
die with a EAGAIN OSError. Ergo, put the tty back in blocking mode
before proceeding.