Mark RemoteCacheStoreTest.testSocketTimeoutForRemoteStore as unstable

* At times, the exception message does not contain
SocketTimeoutException. Judging from the stacktrace on CI, the error
message might contain the following message instead:
"org.infinispan.commons.CacheException: Unable to start cache loaders",
but the SocketTimeoutException is still thrown.
* So, add more information to the assertion and if necessary, convert
it into an assertion that can cope with multiple error messages.