Simplify unit tests listed in Contribution Guide (#2904)

* Simplify unit tests listed in Contribution Guide

* Use long name for option flags

lib/spack/docs/contribution_guide.rst

@@ -75,7 +75,10 @@ This allows you to develop iteratively: make a change, test that change, make
 another change, test that change, etc. To get a list of all available unit
 tests, run:
 
-.. command-output:: spack test --collect-only
+.. command-output:: spack test --list
+
+A more detailed list of available unit tests can be found by running
+``spack test --long-list``.
 
 Unit tests are crucial to making sure bugs aren't introduced into Spack. If you
 are modifying core Spack libraries or adding new functionality, please consider