xfail a test to unblock the snapshot

cc rust-lang#12021

src/test/debug-info/function-arg-initialization.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 // xfail-android: FIXME(#10381)
+// xfail-test: FIXME(#12021)
 
 // This test case checks if function arguments already have the correct value when breaking at the
 // first line of the function, that is if the function prologue has already been executed at the