[clangd] Speed up a slow sleeping testcase.

This testcase runs slowly due to 3.2s of sleeps = 2 + 1 + 0.2s.
After this patch it has 0.55s only.

Reduced by:
 - observed that the last test was bogus: we were sleeping until the queue was
   idle, effectively just a second copy of the first test. This avoids 1s sleep.
 - when waiting for debounce, sleep only until test passes, not for enough
   time to be safe (in practice was 2x debounce time, now 1x debounce time)
 - scaling delays down by a factor of 2 (note: factor of 10 caused bot failures)

Differential Revision: https://reviews.llvm.org/D125103