tests: don't rely on 'false' exit code in test-run-tests.t

On Solaris:
$ sh -c 'false ; echo $?'
255

tests/test-run-tests.t

@@ -18,7 +18,7 b' Multi-line command:'
 
 Return codes before inline python:
 
-  $ false
+  $ sh -c 'exit 1'
   [1]
 
 Doctest commands: