Fix the 'nvm deactivate' test

`nvm deactivate` does not really remove the nvm command, it just remove
the entries from $PATH and $MANPATH previously adde with `nvm use`.

test/fast/Running "nvm deactivate" should unset the nvm environment variables.

@@ -1,6 +1,14 @@
 #!/bin/sh
 
+mkdir -p ../../v0.2.3
+
+die () { echo $@ ; exit 1; }
+
+[[ $PATH != *v0.2.3/*/bin* ]] || echo "WARNING: Unexpectedly found v0.2.3 already active" >&2
+
 . ../../nvm.sh
-nvm &&
+nvm use v0.2.3 &&
+[[ $PATH == *v0.2.3/*/bin* ]] || die "Failed to activate v0.2.3"
+
 nvm deactivate &&
-! nvm
+[[ $PATH != *v0.2.3/*/bin* ]] || die "Failed to deactivate v0.2.3"