Make running perl's tests conditional

Make running perl's tests conditional, one must now specify the
`--run-tests` flag to the `spack install` command in order to run the
tests.

On one system (8 core, 16GB Digital Ocean Droplet), installing without
tests takes 3 minutes, with tests takes 16 minutes.

var/spack/repos/builtin/packages/perl/package.py

@@ -33,7 +33,8 @@ def install(self, spec, prefix):
         configure = Executable('./Configure')
         configure("-des", "-Dprefix=" + prefix)
         make()
-        make("test")
+        if self.run_tests:
+            make("test")
         make("install")
 
         if '+cpanm' in spec: