filesystems/e2fsprogs-core: try to reduce timeout-related pkg-fallout@

The r_corrupt_fs self-test seems to have failed often recently on
various builders with a timeout, which I cannot reproduce. Disable
unless SLOWTESTS is set, and extend timeout from 3 to 10 minutes.

Not bumping PORTREVISION because this does not change the package
contents, only helps a "have package" or "failed build" situation.