tests: make a variable for hg binary location in test-check-py3-commands

The number of which calls in here is starting to get silly.

tests/test-check-py3-commands.t

@@ -3,10 +3,11 b''
 This test helps in keeping a track on which commands we can run on
 Python 3 and see what kind of errors are coming up.
 The full traceback is hidden to have a stable output.
+  $ HGBIN=`which hg`
 
   $ for cmd in version debuginstall ; do
   >   echo $cmd
-  >   $PYTHON3 `which hg` $cmd 2>&1 2>&1 | tail -1
+  >   $PYTHON3 $HGBIN $cmd 2>&1 2>&1 | tail -1
   > done
   version
   warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
@@ -20,7 +21,7 b' The full traceback is hidden to have a s'
   $ cat >> $HGRCPATH <<EOF
   > %include $TESTTMP/included-hgrc
   > EOF
-  $ $PYTHON3 `which hg` version | tail -1
+  $ $PYTHON3 $HGBIN version | tail -1
   *** failed to import extension babar from imaginary_elephant: *: 'imaginary_elephant' (glob)
   warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.