made stress tests conform with this line from spec2: <key> is alphanumeric.

tests/stress/stress_tests.py

@@ -7,7 +7,7 @@ from ...utils.testcase import TestCase
 from ...utils.util import Logger
 from ..helper import KVSMultiClient, KVSTestFixture
 
-def make_random_key(prefix="key-", length=8):
+def make_random_key(prefix="key", length=8):
     chars = string.ascii_lowercase + string.digits
     random_part = ''.join(random.choice(chars) for _ in range(length))
     return f"{prefix}{random_part}"