Document E2E tests

doc/testing.md

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+# Testing
+
+First tests have been set up to cover GPG Mailgate with at least basic test
+that would be easy to run.  The tests are called "end-to-end", meaning that we
+feed some input to GPG Mailgate and inspect the output.
+
+## Running tests
+
+To run tests, use command `make test`.
+
+Tests produce some helpful logs, so inspect contents of `test/logs` directory
+if something goes wrong.
+
+## Key building blocks
+
+- *Test Script* (`test/e2e_test.py`) that orchestrates the other components.
+  It performs test cases described in the *Test Configuration*.  It spawns
+  *Test Mail Relay* and *GPG Mailgate* in appropriate order.
+- *Test Mail Relay* (`test/relay.py`), a simplistic mail daemon that only
+  supports the happy path.  It accepts a mail message and prints it to
+  stdandard output.
+- *Test Configuration* (`test/e2e.ini`) specifies test cases: their input,
+  expected results and helpful documentation.  It also specifies the port that
+  the *Test Mail Relay* should listen on.
+
+## Limitations
+
+Currently tests only check if the message has been encrypted, without
+verifying that the correct key has been used.  That's because we don't know
+(yet) how to have a reproducible encrypted message.  Option
+`--faked-system-time` wasn't enough to produce identical output.