testbench: clear n_faults in string/space/echo_with_long_hole

This happens before the call to Unmap, so that only post-flush faults are
counted (unless the Unmap call ends up causing faults, which'd confuse
things).
testbench: synchronize with test_tid in string/meta/delay_test

This is required to ensure that the string_test_thread() enters a reply/wait
Ipc syscall, and is thus available for a "right now" call.
testbench: string: effect delay regardless of reply success
testbench: string: relax test points in echo_with_long_hole

Also make the diags more explicit for that day when it starts failing again
more reliably.