prefer-regexp-test: Support explicit test conditions

[DMartens]

Description
The rule prefer-regexp-test currently only supports implicit tests for the match (e.g. if (/regex/exec(string))) or boolean casts but not explicit tests (e.g. /regex/.exec(string) !== null).
This should be straightforward to implement in isUseBoolean.
I am willing to submit a PR for this.

[ota-meshi]

Thank you for submitting this report!
Feel free to open a PR!