test: fix flaky 'sigterm during box.cfg' test

The test sends SIGTERM to the process while it is writing a
snapshot. Using `g.server.process:kill('TERM')`.

On a next line the test was doing `g.server:stop()` which also
sends SIGTERM. Apparently, the second SIGTERM sometimes killed the
server immediately.

It usually happened inside fiber_free(). Sometimes right after the
file tt_exit_file.txt.inprogress was created but before it was
renamed to tt_exit_file.txt.

The reason is in signal_free() being called quite early. After
that any signal becomes fatal and is not caught by the handlers.

The patch also increases on-shutdown timeout not to depend on it
for certain. Although it shouldn't be a problem right now.

Follow-up #7743

NO_DOC=test
NO_CHANGELOG=test