Use patch.patch() when checking for patch availability.
The test is closer to what we want to know. It also makes sense when you deal with patched versions of patch() as most people do under win32.
#!/bin/sh
echo "[extensions]" >> $HGRCPATH
echo "mq=" >> $HGRCPATH
hg init a
cd a
echo 'base' > base
hg ci -Ambase -d '1 0'
hg qnew a
hg qnew b
hg qnew c
hg qdel c
hg qpop
hg qdel c
hg qseries
ls .hg/patches
hg qpop
hg qdel -k b
ls .hg/patches
hg qdel -r a
hg qapplied
hg log --template '{rev} {desc}\n'
hg qnew d
hg qnew e
hg qnew f
hg qdel -r e
hg qdel -r qbase:e
hg qapplied
hg log --template '{rev} {desc}\n'