chore(gulp-plugins): enable consumers to use mocha's "exit" flag
chore(gulp-plugins): enable consumers to use mocha's "exit" flag

unit tests _should_ not need `--exit`, but this is real life, so things aren't perfect.

boilerplate configs can pass `{test: {exit: true}}` to cause Mocha to force-exit.

note that this is enabled by default for e2e tests (which _also_ shouldn't be necessary, but one thing at a time...)

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