test: add a function to run Python tests

The new test_python() function makes writing Python tests a little easier:
- it sets the environment variables as needed
- it redirects stdout to the OUTPUT file (like test_emacs()).

This commit also declares python as an external prereq.

The stdout redirection is required to avoid trouble when running commands like
"python 'script' | sort > OUTPUT": in such a case, any error due to a missing
external prereq would be "swallowed" by sort, resulting to a failed test instead
of a skipped one.

test/python

@@ -5,9 +5,7 @@ test_description="python bindings"
 add_email_corpus
 
 test_begin_subtest "compare thread ids"
-LD_LIBRARY_PATH=$TEST_DIRECTORY/../lib \
-PYTHONPATH=$TEST_DIRECTORY/../bindings/python \
-python <<EOF | sort > OUTPUT
+test_python <<EOF
 import notmuch
 db = notmuch.Database(mode=notmuch.Database.MODE.READ_WRITE)
 q_new = notmuch.Query(db, 'tag:inbox')
@@ -15,5 +13,5 @@ for t in q_new.search_threads():
     print t.get_thread_id()
 EOF
 notmuch search --output=threads tag:inbox | sed s/^thread:// | sort > EXPECTED
-test_expect_equal_file OUTPUT EXPECTED
+test_expect_equal_file <(sort OUTPUT) EXPECTED
 test_done

test/test-lib.sh

@@ -919,6 +919,14 @@ test_emacs () {
 	emacsclient --socket-name="$EMACS_SERVER" --eval "(progn $@)"
 }
 
+test_python() {
+	export LD_LIBRARY_PATH=$TEST_DIRECTORY/../lib
+	export PYTHONPATH=$TEST_DIRECTORY/../bindings/python
+
+	(echo "import sys; _orig_stdout=sys.stdout; sys.stdout=open('OUTPUT', 'w')"; cat) \
+		| python -
+}
+
 # Creates a script that counts how much time it is executed and calls
 # notmuch.  $notmuch_counter_command is set to the path to the
 # generated script.  Use notmuch_counter_value() function to get the
@@ -1180,3 +1188,4 @@ test_declare_external_prereq emacs
 test_declare_external_prereq emacsclient
 test_declare_external_prereq gdb
 test_declare_external_prereq gpg
+test_declare_external_prereq python