Split all the e2e tests in their own files

Some duplication is avoided, but most importantly: it’s much much cleaner,
easier to find, functions have been cleaned up (functools.partial usages are
now hidden), etc.

tests/end_to_end/scenarios/basic_handshake_success.py

@@ -0,0 +1,5 @@
+from scenarios import *
+
+scenario = (
+    sequences.handshake(),
+)