add failing test for splat parameter flow

Author: erik-krogh

ruby/ql/test/library-tests/dataflow/params/params_flow.rb

@@ -57,4 +57,10 @@ def posargs(p1, *posargs)
 posargs(taint(23), *args)
 
 args = [taint(24), taint(25)]
-posargs(*args)
+posargs(*args)
+
+args = taint(26)
+def splatstuff(*x)
+    sink x[0] # $ MISSING: hasValueFlow=26
+end
+splatstuff(*args)