Spack tests no longer clutter var/spack/stage

- Tests use a session-scoped mock stage directory so as not to interfere
  with the real install.

- Every test is forced to clean up after itself with an additional check.
  We now automatically assert that no new files have been added to
  `spack.stage_path` during each test.

  - This means that tests that fail installs now need to clean up their
    stages, but in all other cases the check is useful.

var/spack/repos/builtin.mock/packages/canfail/package.py

@@ -33,11 +33,9 @@ class Canfail(Package):
 
     version('1.0', '0123456789abcdef0123456789abcdef')
 
+    succeed = False
+
     def install(self, spec, prefix):
-        try:
-            succeed = getattr(self, 'succeed')
-            if not succeed:
-                raise InstallError("'succeed' was false")
-            touch(join_path(prefix, 'an_installation_file'))
-        except AttributeError:
-            raise InstallError("'succeed' attribute was not set")
+        if not self.succeed:
+            raise InstallError("'succeed' was false")
+        touch(join_path(prefix, 'an_installation_file'))