Adds a :vuln_test option to BES, just like in BAP.

I needed this to run a custom JS check for the Android
webview vuln when the exploit is served straight
through BES. The check already existed when using BAP,
so I tried to preserve that syntax, and also added a
:vuln_test_error as an optional error message.

This commit also does some mild refactoring of un-
useful behavior in BES.
Remove un-truthy doc string.
Land #3934 - New :vuln_test option to BES