test: add mustCall() to test-inspector-contexts

In test-inspector-contexts, if mainContextPromise is modified such that
the `method` argument is changed to something invalid, the test still
passes and the second part of the test never runs. Use
`common.mustCall()` to cause the test to fail if the second part of the
test does not run.

Refs: https://github.com/nodejs/node/issues/30519#issuecomment-558476839

PR-URL: https://github.com/nodejs/node/pull/30649


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>