Add a comment indicating that the btc devs don't want a warning fixed #8005

The src/test/scheduler_tests.cpp test has been disabled since v0.9.0rc2-4332-g8f0d79e, since then it's been warning about the MicroSleep() function being unused, e.g. on GCC 4.9.2-10:

test/scheduler_tests.cpp:32:13: warning: ‘void
scheduler_tests::MicroSleep(uint64_t)’ defined but not used
[-Wunused-function]

The bitcoin developers don't want this warning fixed, and are instead using it as a reminder to fix the test. Since this is a rather unorthodox use of compiler warnings add a comment about this so people who build bitcoin and notice compiler warnings don't try to submit patches for this one.

See #8003 and #7169 for past attempts to fix this warning which have been rejected.

I'd rather have the test fixed, or replaced by a test without the race condition.

But given no one is doing that at the moment, this warning is fine with me.

@laanwj Yes it's clear from the various refused pull requests that you want the test fixed and want to keep this warning. This pull request is not orthogonal to that, but is rather for noting that this "compiler warning as a reminder" shouldn't be fixed.

I agree. On a higher level the philosophy of 'fixing compiler warnings' is wrong. It's fighting symptoms, not the underlying cause. It's like 'fixing' a building by papering over cracks. In a way it is compiler's own fault for crying wolf too many times, warning about silly things, so it's understandable, but I've often seen cases where people bash in compiler warnings without even considering the problem they're warning about - which were pointing out a legit bug.

No longer necessary after #8016, which fixes the underlying issue.