Use the standard session and options mock object together with the
new composer mock object to simulate a successful compose:
the mock composer will accept the request and also status calls
for it, which will immediately return with success. The plugin
should then return and indicate a successful compose.
Use httpretty to mock composer responses. Create a helper method
to configure a session object that should result in a valid
compose request. Simulate a bad request by asking for an arch
that the mock composer will refuse and check the correct exception
is thrown in the builder plugin.
Create a new plugintest.PluginTest class that shares the plugin
loading code that is common to all plugin testing. Adapt the
existing (hub, builder) tests.
Also correct the name for test_builder.TestHubPlugin to
TestBuilderPlugin.
