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/e2e-test

@@ -216,7 +216,7 @@ for run in $server_tests; do
 	test_error "tmux failure on test %s\n" "$run"
     fi
     # Check for mosh failures
-    if ! grep -q "%%% exitstatus: 0 %%%" "${test_dir}/${run}.tmux.log"; then
+    if ! grep -q "@@@ exitstatus: 0 @@@" "${test_dir}/${run}.tmux.log"; then
 	test_error "mosh-client had non-zero exitstatus\n"
     fi