tests: Rename test_narrow.py to test_message_fetch.py.

This commit renames test_narrow.py and also all its references to
test_message_fetch.py.

tools/ci/Dockerfile.template

@@ -48,7 +48,7 @@ RUN apt-get update \
 # Set the locale, together with the locale-related steps above.
 # It's not entirely clear why, but alternatives that don't work include
 #  * using `C.UTF-8` instead of `en_US.UTF-8`, here and above
-#    (causes mysterious failures in zerver.tests.test_narrow)
+#    (causes mysterious failures in zerver.tests.test_message_fetch)
 #  * skipping the /etc/default/locale step above (ditto)
 #  * skipping this ENV instruction (causes provision to fail,
 #    because Python tries to use the `ascii` codec)

tools/linter_lib/custom_check.py

@@ -35,7 +35,7 @@ FILES_WITH_LEGACY_SUBJECT = {
 
     # This has lots of query data embedded, so it's hard
     # to fix everything until we migrate the DB to "topic".
-    'zerver/tests/test_narrow.py',
+    'zerver/tests/test_message_fetch.py',
 }
 
 shebang_rules: List["Rule"] = [