Fix stop timing flake by widening upper bound

The `podman stop - basic test` flakes in CI because $SECONDS is
integer-precision: a ~14.6s stop rounds to delta_t=15 and fails
the -le 14 check. Widen the upper bound to 18s, which still catches
real regressions while tolerating slow CI nodes and remote-mode
socket overhead.

Signed-off-by: Jan Rodák <hony.com@seznam.cz>

test/system/050-stop.bats

@@ -31,7 +31,7 @@ load helpers
     # exactly 10 seconds. Give it some leeway.
     delta_t=$(( $t1 - $t0 ))
     assert $delta_t -gt  8 "podman stop: ran too quickly!"
-    assert $delta_t -le 14 "podman stop: took too long"
+    assert $delta_t -le 18 "podman stop: took too long"
 
     run_podman rm $cid
 }