setup-env.sh: make setup scripts work with set -u

- Add set -u to the setup-env.sh test script

- Refactor lines in setup-env.sh that tested potentially undefined
  variables to use the `[ -z ${var+x} ]` construct

share/spack/qa/setup-env-test.sh

@@ -163,6 +163,9 @@ unuse() {
 # Make sure no environment is active
 unset SPACK_ENV
 
+# fail with undefined variables
+set -u
+
 # Source setup-env.sh before tests
 .  share/spack/setup-env.sh