tests: use pdiff instead of diff for portability stable

tests: use pdiff instead of diff for portability

"diff" command might cause redundant message, "No differences
encountered" on Solaris for example. But suppressing option like "-q"
isn't portable, because POSIX specification doesn't define it.

pdiff script was introduced by b59ef0c21405 to stabilize output of
standard diff command on each platforms.

--- a/tests/test-cache-abuse.t	Mon Jul 31 13:10:19 2017 +0900
+++ b/tests/test-cache-abuse.t	Mon Jul 31 13:16:34 2017 +0900
@@ -47,21 +47,21 @@
   >  echo bad > $CACHE
   >  test -z "$CLEAN" || $CLEAN
   >  hg $CMD > after
-  >  diff -u before after || echo "*** overwrite corruption"
+  >  "$RUNTESTDIR/pdiff" before after || echo "*** overwrite corruption"
   >  echo corruption >> $CACHE
   >  test -z "$CLEAN" || $CLEAN
   >  hg $CMD > after
-  >  diff -u before after || echo "*** append corruption"
+  >  "$RUNTESTDIR/pdiff" before after || echo "*** append corruption"
   >  rm $CACHE
   >  mkdir $CACHE
   >  test -z "$CLEAN" || $CLEAN
   >  hg $CMD > after
-  >  diff -u before after || echo "*** read-only corruption"
+  >  "$RUNTESTDIR/pdiff" before after || echo "*** read-only corruption"
   >  test -d $CACHE || echo "*** directory clobbered"
   >  rmdir $CACHE
   >  test -z "$CLEAN" || $CLEAN
   >  hg $CMD > after
-  >  diff -u before after || echo "*** missing corruption"
+  >  "$RUNTESTDIR/pdiff" before after || echo "*** missing corruption"
   >  test -f $CACHE || echo "not rebuilt"
   > }