Improve benchmarks and exclude from distro

Change-Id: If70fa19418ccfd339695997944ac34350742319a
diff --git a/MANIFEST.SKIP b/MANIFEST.SKIP
index f8efe15..c2a6eb1 100755
--- a/MANIFEST.SKIP
+++ b/MANIFEST.SKIP
@@ -1,5 +1,6 @@
 # Skip real corpus regression files and tests
 ^t/real/
+^xt/
 
 # Makemaker generated files and dirs.
 ^MANIFEST\.