make the leak tests release-only

diff --git a/t/20-leaks.t b/xt/release/20-leaks.t
similarity index 100%
rename from t/20-leaks.t
rename to xt/release/20-leaks.t

diff --git a/t/21-leaks-debug.t b/xt/release/21-leaks-debug.t
similarity index 100%
rename from t/21-leaks-debug.t
rename to xt/release/21-leaks-debug.t