fix: Remove WEB_EXT_* env vars not supported by the selected web-ext command

[rpl]

This PR contains another possible approach to fix #793 (as an alternative to disabling the yargs strict parsing as in #1810 )

• add new unit tests to cover the changes (and fix coveralls failure due to decreased code coverage)
• add an integration test (to make sure the fix does work as expected when the options and environment vars are actually parsed and validated by yargs).

[coveralls]

Coverage remained the same at 100.0% when pulling 18f2118 on rpl:fix/yargs-env-vars-parsing-issue into 333b29b on mozilla:master.

[rpl]

@Rob--W this PR has rebased on top of #1813 and the tests still pass as expected.

should be ready for one last look

[apiraino]

thanks @rpl ! 👍