Gather together usefulness tests

I took most tests that were testing only for match exhaustiveness,
pattern refutability or match arm reachability, and put them in
the same test folder.

src/test/ui/pattern/usefulness/match-byte-array-patterns-2.rs

@@ -0,0 +1,13 @@
+fn main() {
+    let buf = &[0, 1, 2, 3];
+
+    match buf { //~ ERROR non-exhaustive
+        b"AAAA" => {}
+    }
+
+    let buf: &[u8] = buf;
+
+    match buf { //~ ERROR non-exhaustive
+        b"AAAA" => {}
+    }
+}