tests: improve robustness

Replace some blind timeouts with actual synchronization.
Improve performance on some slow tests.
Tweak some of the remaining timeouts.

This still isn't good enough to get 'make -j9 check'
working reliably on Cygwin for me; I think some Cygwin/
Windows scheduling issues remain.

src/tests/emulation-multiline-scroll.test

@@ -32,7 +32,7 @@ baseline()
 	    # delays so that each iteration percolates to the client
 	    # by itself.  On the other hand, that makes the test take
 	    # a long time.  Compromise on .1 second.
-	    (sleep .1 || sleep 1) > /dev/null 2>&1
+	    sleepf
 	done
     done
 }