Fix[bmq]: Flush output on test and contract failures

[hallfox]

We're seeing failures from tests without any of the context we'd expect (e.g. a bsls assertion failure without line number info). To fix, we should explicitly flush stdout/stderr depending on which mode we're failing in.

[dorjesinpo]

test_shutdown_upstream_proxy[virtual_fsm] failure is irrelevant and can be ignored

mqbc_incoreclusterstateledger.t, failure is irrelevant. It is output proves the PR is working
Error /home/runner/work/blazingmq/blazingmq/src/groups/mqb/mqbc/mqbc_incoreclusterstateledger.t.cpp(110): 'cslIter.header().recordType()' (UPDATE) == 'recordType' (SNAPSHOT) (failed)

We get file, line, expression now!

[dorjesinpo]

Thank you