Move compiletest meta tests to a separate directory

src/test/ui/meta/expected-error-correct-rev.a.stderr

@@ -1,5 +1,5 @@
 error[E0308]: mismatched types
-  --> $DIR/meta-expected-error-correct-rev.rs:7:18
+  --> $DIR/expected-error-correct-rev.rs:7:18
    |
 LL |     let x: u32 = 22_usize;
    |            ---   ^^^^^^^^ expected `u32`, found `usize`

src/test/ui/meta/expected-error-correct-rev.rs

@@ -1,6 +1,6 @@
 // revisions: a
 
-// Counterpart to `meta-expected-error-wrong-rev.rs`
+// Counterpart to `expected-error-wrong-rev.rs`
 
 #[cfg(a)]
 fn foo() {

src/test/ui/meta/revision-ok.rs

@@ -1,5 +1,5 @@
 // Meta test for compiletest: check that when we give the right error
-// patterns, the test passes. See all `meta-revision-bad.rs`.
+// patterns, the test passes. See all `revision-bad.rs`.
 
 // run-fail
 // revisions: foo bar