Add unit tests for dependencies being patched by parent

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

@@ -14,5 +14,7 @@ class Libelf(Package):
     version('0.8.12', 'e21f8273d9f5f6d43a59878dc274fec7')
     version('0.8.10', '9db4d36c283d9790d8fa7df1f4d7b4d9')
 
+    patch('local.patch', when='@0.8.10')
+
     def install(self, spec, prefix):
         touch(prefix.libelf)

var/spack/repos/builtin.mock/packages/patch-several-dependencies/package.py

@@ -15,10 +15,14 @@ class PatchSeveralDependencies(Package):
     version('2.0', '0123456789abcdef0123456789abcdef')
     version('1.0', '0123456789abcdef0123456789abcdef')
 
+    variant('foo', default=False,
+            description='Forces a version on libelf')
+
     # demonstrate all the different ways to patch things
 
     # single patch file in repo
     depends_on('libelf', patches='foo.patch')
+    depends_on('libelf@0.8.10', patches='foo.patch', when='+foo')
 
     # using a list of patches in one depends_on
     depends_on('libdwarf', patches=[