test-client: increase context in pexecpt failure for debugging

When a pexpect check fails, we want to see the full content of the
buffer, so we can better see where it went wrong. Increase the context
that is printed in the error message.

src/tests/client/test-client.py

@@ -1009,6 +1009,8 @@ class TestNmClient(unittest.TestCase):
 
         pexp = pexpect.spawn(argv[0], argv[1:], timeout=10, env=env)
 
+        pexp.str_last_chars = 100000
+
         typ = collections.namedtuple("CallPexpect", ["pexp", "valgrind_log"])
         return typ(pexp, valgrind_log)