Disable BackpressureObjectCounterTests 49

[cody-littley]

A fix was backported into 0.49 that improved back pressure performance. Unfortunately this fix also make some unit tests flaky.

The cause of the flakyness is well understood and has been fixed in develop. Since those fixes were done over many PRs, back porting the test fixes is non-trivial. Let's just disable the flaky tests in this branch. We shouldn't be making any additional changes to the covered code in 0.49 (if we do we need to re-enable and fix the tests).