test: increase test timeout to prevent flakiness

This increases the waiting time for each event from 500 to 750 ms. The former timeout could be hit on very slow machines with high load.

See e.g. https://ci.nodejs.org/job/node-test-commit-linux-containered/17948/nodes=ubuntu1804_sharedlibs_zlib_x64/testReport/junit/(root)/test/parallel_test_repl/

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines