tests: add tests for spack extensions command

- add tests for `spack extensions`
- refactor `test_activations` test to use real extensions.

var/spack/repos/builtin.mock/packages/perl-extension/package.py

@@ -15,14 +15,9 @@ class PerlExtension(PerlPackage):
     version('1.0', 'hash-extension-1.0')
     version('2.0', 'hash-extension-2.0')
 
+    extends("perl")
+
     def install(self, spec, prefix):
         mkdirp(prefix.bin)
         with open(os.path.join(prefix.bin, 'perl-extension'), 'w+') as fout:
             fout.write(str(spec.version))
-
-    # Give the package a hook to set the extendee spec
-    extends_spec = 'perl'
-
-    @property
-    def extendee_spec(self):
-        return self.extends_spec

var/spack/repos/builtin.mock/packages/py-extension1/package.py

@@ -20,9 +20,4 @@ def install(self, spec, prefix):
         with open(os.path.join(prefix.bin, 'py-extension1'), 'w+') as fout:
             fout.write(str(spec.version))
 
-    # Give the package a hook to set the extendee spec
-    extends_spec = 'python'
-
-    @property
-    def extendee_spec(self):
-        return self.extends_spec
+    extends('python')

var/spack/repos/builtin.mock/packages/py-extension2/package.py

@@ -13,6 +13,7 @@ class PyExtension2(PythonPackage):
     homepage = "http://www.example.com"
     url      = "http://www.example.com/extension2-1.0.tar.gz"
 
+    extends("python")
     depends_on('py-extension1', type=('build', 'run'))
 
     version('1.0', 'hash-extension2-1.0')
@@ -21,10 +22,3 @@ def install(self, spec, prefix):
         mkdirp(prefix.bin)
         with open(os.path.join(prefix.bin, 'py-extension2'), 'w+') as fout:
             fout.write(str(spec.version))
-
-    # Give the package a hook to set the extendee spec
-    extends_spec = 'python'
-
-    @property
-    def extendee_spec(self):
-        return self.extends_spec