add spack test list --all (#22032)

`spack test list` will show you which *installed* packages can be tested
but it won't show you which packages have tests.

- [x] add `spack test list --all` to show which packages have test methods
- [x] update `has_test_method()` to handle package instances *and*
      package classes.

share/spack/spack-completion.bash

@@ -1548,7 +1548,7 @@ _spack_test_run() {
 }
 
 _spack_test_list() {
-    SPACK_COMPREPLY="-h --help"
+    SPACK_COMPREPLY="-h --help -a --all"
 }
 
 _spack_test_find() {