Tests fail if directory has a space

[shadowspawn]

 FAIL  tests/command.default.test.js
 FAIL  tests/command.executableSubcommand.lookup.test.js

Test Suites: 2 failed, 42 passed, 44 total
Tests:       15 failed, 356 passed, 371 total

Example of unsafe code in tests:

const { stdout } = await execAsync(`node ${pm} --an-option`);

Uncovered when suggested adding Commander back into citgm:

• Commander hopefully no longer flaky nodejs/citgm#838

[shadowspawn]

Fix released in 6.2.1