test: move timer-dependent test to sequential

Timer-dependent tests fail frequently on certain platforms in CI when
run in parallel with other tests, likely due to competition for
resources. Move test-repl-timeout-throw to sequential to avoid this
problem. Also did some minor refactoring (var->const and more use of
assert.strictEqual of looser assertions).

PR-URL: https://github.com/nodejs/node/pull/9431


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Johan Bergstrom <bugs@bergstroem.nu>