test(frontmatter): Rename tests to make coverage more obvious

When working on the stabilization report (rust-lang#148051),
I found it annoying to determine what cases were covered because areas
of the frontmatter feature were either not in the file name or in
inconsistent locations.

This moves the area of frontmatter to the start of the file name and the
moves to more specific the later in the file name so coverage is easier
to see.

Author: epage

tests/ui/frontmatter/frontmatter-contains-whitespace.rs renamed to tests/ui/frontmatter/content-contains-whitespace.rs

@@ -1,5 +1,5 @@
 error: extra characters after frontmatter close are not allowed
-  --> $DIR/extra-after-end.rs:2:1
+  --> $DIR/fence-close-extra-after.rs:2:1
    |
 LL | ---cargo
    | ^^^^^^^^

tests/ui/frontmatter/frontmatter-non-lexible-tokens.rs renamed to tests/ui/frontmatter/content-non-lexible-tokens.rs

@@ -1,19 +1,19 @@
 error: unclosed frontmatter
-  --> $DIR/frontmatter-whitespace-2.rs:1:1
+  --> $DIR/fence-indented-mismatch.rs:1:1
    |
 LL | / ---cargo
 ...  |
 LL | |
    | |_^
    |
 note: frontmatter opening here was not closed
-  --> $DIR/frontmatter-whitespace-2.rs:1:1
+  --> $DIR/fence-indented-mismatch.rs:1:1
    |
 LL | ---cargo
    | ^^^
 
 warning: use of a double negation
-  --> $DIR/frontmatter-whitespace-2.rs:9:6
+  --> $DIR/fence-indented-mismatch.rs:9:6
    |
 LL |     ---x
    |      ^^^