Rename test files to satisfy tidy.

To get past the annoying "must begin with a descriptive name, consider
`{reason}-issue-121431.rs`" test.

I find choosing a location and name is the worst part of adding a new
ui test, and this just makes it harder.

tests/ui/generics/issue-121418.stderr → tests/ui/generics/span-bug-issue-121418.stderr

@@ -1,5 +1,5 @@
 error: inherent impls cannot be `const`
-  --> $DIR/issue-121418.rs:7:12
+  --> $DIR/span-bug-issue-121418.rs:7:12
    |
 LL | impl const dyn T {
    |      ----- ^^^^^ inherent impl for this type
@@ -9,7 +9,7 @@ LL | impl const dyn T {
    = note: only trait implementations may be annotated with `const`
 
 error[E0207]: the const parameter `host` is not constrained by the impl trait, self type, or predicates
-  --> $DIR/issue-121418.rs:7:6
+  --> $DIR/span-bug-issue-121418.rs:7:6
    |
 LL | impl const dyn T {
    |      ^^^^^ unconstrained const parameter

tests/ui/lowering/issue-121431.stderr → tests/ui/lowering/span-bug-issue-121431.stderr

@@ -1,5 +1,5 @@
 error[E0405]: cannot find trait `CallbackMarker` in this scope
-  --> $DIR/issue-121431.rs:1:21
+  --> $DIR/span-bug-issue-121431.rs:1:21
    |
 LL | fn bug<T>() -> impl CallbackMarker< Item = [(); { |_: &mut ()| 3; 4 }] > {}
    |                     ^^^^^^^^^^^^^^ not found in this scope

tests/ui/traits/issue-121414.stderr → tests/ui/traits/span-bug-issue-121414.stderr

@@ -1,13 +1,13 @@
 error[E0261]: use of undeclared lifetime name `'f`
-  --> $DIR/issue-121414.rs:5:22
+  --> $DIR/span-bug-issue-121414.rs:5:22
    |
 LL | impl<'a> Bar for Foo<'f> {
    |      -               ^^ undeclared lifetime
    |      |
    |      help: consider introducing lifetime `'f` here: `'f,`
 
 error: implementation of `Bar` is not general enough
-  --> $DIR/issue-121414.rs:9:4
+  --> $DIR/span-bug-issue-121414.rs:9:4
    |
 LL | fn test()
    |    ^^^^ implementation of `Bar` is not general enough

tests/ui/typeck/issue-121410.stderr → tests/ui/typeck/span-bug-issue-121410.stderr

@@ -1,5 +1,5 @@
 error[E0308]: mismatched types
-  --> $DIR/issue-121410.rs:9:13
+  --> $DIR/span-bug-issue-121410.rs:9:13
    |
 LL |         let (_, t2) = foo.x;
    |             ^^^^^^^   ----- this expression has type `String`