Update testing documentation

- Explain how to specify Python binary path used during tests.

- Mention test logs.

doc/testing.md

@@ -11,6 +11,10 @@ To run tests, use command `make test` or `make unittest`.
 Tests produce some helpful logs, so inspect contents of `test/logs` directory
 if something goes wrong.
 
+If your system's Python binary isn't found in your `$PATH` or you want to use
+a specific binary, use make's macro overriding: `make test
+PYTHON=/path/to/python`.
+
 ## Key building blocks
 
 - *Test Script* (`test/e2e_test.py`) that orchestrates the other components.
@@ -29,3 +33,8 @@ 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.
+
+## Troubleshooting
+
+When things go wrong, be sure to study `test/logs/e2e.log` and
+`test/logs/gpg-mailgate.log` files -- they contain some useful information.