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/librustc_mir/hair/pattern/_match.rs

@@ -1,3 +1,6 @@
+/// Note: most tests relevant to this file can be found (at the time of writing)
+/// in src/tests/ui/pattern/usefulness.
+///
 /// This file includes the logic for exhaustiveness and usefulness checking for
 /// pattern-matching. Specifically, given a list of patterns for a type, we can
 /// tell whether:

src/test/ui/pattern/usefulness/match-arm-statics-2.stderr

@@ -1,5 +1,5 @@
 error[E0004]: non-exhaustive patterns: `(true, false)` not covered
-  --> $DIR/match-argm-statics-2.rs:17:11
+  --> $DIR/match-arm-statics-2.rs:17:11
    |
 LL |     match (true, false) {
    |           ^^^^^^^^^^^^^ pattern `(true, false)` not covered
@@ -7,7 +7,7 @@ LL |     match (true, false) {
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
 error[E0004]: non-exhaustive patterns: `Some(Some(West))` not covered
-  --> $DIR/match-argm-statics-2.rs:29:11
+  --> $DIR/match-arm-statics-2.rs:29:11
    |
 LL |     match Some(Some(North)) {
    |           ^^^^^^^^^^^^^^^^^ pattern `Some(Some(West))` not covered
@@ -15,7 +15,7 @@ LL |     match Some(Some(North)) {
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
 error[E0004]: non-exhaustive patterns: `Foo { bar: Some(North), baz: NewBool(true) }` not covered
-  --> $DIR/match-argm-statics-2.rs:48:11
+  --> $DIR/match-arm-statics-2.rs:48:11
    |
 LL | / struct Foo {
 LL | |     bar: Option<Direction>,