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.stderr

@@ -0,0 +1,19 @@
+error[E0004]: non-exhaustive patterns: `&[_, _, _, _]` not covered
+  --> $DIR/match-byte-array-patterns-2.rs:4:11
+   |
+LL |     match buf {
+   |           ^^^ pattern `&[_, _, _, _]` not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+
+error[E0004]: non-exhaustive patterns: `&[]`, `&[_]`, `&[_, _]` and 3 more not covered
+  --> $DIR/match-byte-array-patterns-2.rs:10:11
+   |
+LL |     match buf {
+   |           ^^^ patterns `&[]`, `&[_]`, `&[_, _]` and 3 more not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0004`.