Skip vendoring libffi/libmemcached on Heroku-16 (#465)

* Add a test for the cryptography (cffi) compile step

Since it's currently untested.

* Skip vendoring libffi/libmemcached on Heroku-16

Unlike for Cedar-14, Heroku-16 comes with these packages pre-installed:
https://github.com/heroku/stack-images/blob/5a341970cfc1f201014262ad64c3b3e47514f663/heroku-16/installed-packages.txt#L111
https://github.com/heroku/stack-images/blob/5a341970cfc1f201014262ad64c3b3e47514f663/heroku-16/installed-packages.txt#L172

As such the build scripts had already been made a no-op on Heroku-16:
https://github.com/heroku/heroku-buildpack-python/blob/fedae5ceda5a42f594012f911c3808dd5dc6fd9e/builds/libraries/vendor/libffi#L6-L9
https://github.com/heroku/heroku-buildpack-python/blob/fedae5ceda5a42f594012f911c3808dd5dc6fd9e/builds/libraries/vendor/libmemcache#L6-L9

...meaning the Heroku-16 archives for them on S3 contain zero files.

However until now, the buildpack was still unnecessarily downloading
and extracting these empty archives - and not just on the first compile
(like on cedar-14), but every compile since the directory check will
never succeed.

test/fixtures/cffi/requirements.txt

@@ -0,0 +1 @@
+cffi

test/run

@@ -47,6 +47,12 @@ testPsycopg2() {
   assertCapturedSuccess
 }
 
+testCffi() {
+  compile "cffi"
+  assertCaptured "cffi"
+  assertCapturedSuccess
+}
+
 testPylibmc() {
   compile "pylibmc"
   assertCaptured "pylibmc"