Make alias spec test less sensitive to commands on $PATH.

Addresses issue #22.

spec/alias.test.sh

@@ -111,11 +111,11 @@ e X

### Alias detection happens before expansion
shopt -s expand_aliases  # bash requires this
alias e='echo'
cmd=e
e X
alias echo_alias_='echo'
cmd=echo_alias_
echo_alias_ X
$cmd X
e status=$?
echo_alias_ status=$?
## STDOUT:
X
status=127