This adds tests for issues brought up in bug 231393, bug 264610, bug 302575 and bug 1129564,
all of which fed into the current implementation of userTypedClear/userTypedValue. I intend
to move us away from userTypedClear, but I'm keen not to regress any of these issues, so
I'm adding automated tests to ensure that doesn't happen.
MozReview-Commit-ID: 1up2MIXzkzG