Disable some timing-sensitive tests in mixed version mode

These depend on credentials_obfuscation being initialized/seeded,
and the timing seems to be different on 3.9.x or the way we run
those nodes as part of Bazel suites.

The same suite passes 10 times in a row with 6 shards when
all nodes are 3.10.x ones.

(cherry picked from commit 37c48d6)

Author: michaelklishin