tests: use strip extension instead of mq where it makes sense

When a test only uses 'hg strip', it no longer needs to load MQ.

tests/test-impexp-branch.t

@@ -1,5 +1,5 b''
   $ echo '[extensions]' >> $HGRCPATH
-  $ echo 'mq =' >> $HGRCPATH
+  $ echo 'strip =' >> $HGRCPATH
 
   $ cat >findbranch.py <<EOF
   > import re, sys

tests/test-lfconvert.t

@@ -5,7 +5,7 b''
   > largefiles =
   > share =
   > graphlog =
-  > mq =
+  > strip =
   > convert =
   > [largefiles]
   > minsize = 0.5

tests/test-strip-cross.t

@@ -1,7 +1,7 b''
 test stripping of filelogs where the linkrev doesn't always increase
 
   $ echo '[extensions]' >> $HGRCPATH
-  $ echo 'hgext.mq =' >> $HGRCPATH
+  $ echo 'hgext.strip =' >> $HGRCPATH
   $ hg init orig
   $ cd orig
   $ commit()