Fix: Omit flaky performance test on single-core and for CI

[mathiasrw]

Working on making the travis CI test smooth again.

---

This change is 

[mathiasrw]

This fixed the builds, but even the test is all good now it hangs in travis-ci for windows. This is out of the scope of this PR to fix.

Hanging even after yarn test has exited with 0: https://travis-ci.org/github/parallel-js/parallel.js/jobs/687001993

[MaXFalstein]

The command "yarn test" exited with 0.

store build cache
changes detected (content changed, file is created, or file is deleted):\nC:\Users\travis\build\parallel-js\parallel.js\node_modules\.bin\is-ci
C:\Users\travis\build\parallel-js\parallel.js\node_modules\.bin\is-ci.cmd
C:\Users\travis\build\parallel-js\parallel.js\node_modules\.bin\mkdirp
C:\Users\travis\build\parallel-js\parallel.js\node_modules\.bin\mkdirp.cmd
C:\Users\travis\build\parallel-js\parallel.js\node_modules\.bin\node-which
C:\Users\travis\build\parallel-js\parallel.js\node_modules\.bin\node-which.cmd
C:\Users\travis\build\parallel-js\parallel.js\node_modules\.bin\parser
C:\Users\travis\build\parallel-js\parallel.js\node_modules\.bin\parser.cmd
C:\Users\travis\build\parallel-js\parallel.js\node_modules\.bin\semver
C:\Users\travis\build\parallel-js\parallel.js\node_modules\.bin\semver.cmd
C:\Users\travis\build\parallel-js\parallel.js\node_modules\.bin\which
C:\Users\travis\build\parallel-js\parallel.js\node_modules\.bin\which.cmd
C:\Users\travis\build\parallel-js\parallel.js\node_modules\.yarn-integrity
C:\Users\travis\build\parallel-js\parallel.js\n...
changes detected, packing new archive
gzip: warning: GZIP environment variable is deprecated; use an alias or script
uploading fixCI/cache--windows-1809-containers-e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855--node-node.tgz
cache uploaded

cahce uploaded is successful despite changes detected.

Done. Your build exited with 0.
No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#build-times-out-because-no-output-was-received
The build has been terminated

This appears to be an early access Travis CI Windows hang.
Travis log.txt provides the raw log but it does not give us any information about the hang.