chore(core): Add test for `addExceptionMechanism` (#4073)
chore(core): Add test for `addExceptionMechanism` (#4073)

Extracted from https://github.com/getsentry/sentry-javascript/pull/4068, to ensure that it doesn't break current behavior.
getsentry/raven-js,getsentry/raven-js,getsentry/raven-js,getsentry/raven-js