Move some tests from rustc_expand to rustc_parse.

There are some test cases involving `parse` and `tokenstream` and
`mut_visit` that are located in `rustc_expand`. Because it used to be
the case that constructing a `ParseSess` required the involvement of
`rustc_expand`. However, since #64197 merged (a long time ago)
`rust_expand` no longer needs to be involved.

This commit moves the tests into `rustc_parse`. This is the optimal
place for the `parse` tests. It's not ideal for the `tokenstream` and
`mut_visit` tests -- they would be better in `rustc_ast` -- but they
still rely on parsing, which is not available in `rustc_ast`. But
`rustc_parse` is lower down in the crate graph and closer to `rustc_ast`
than `rust_expand`, so it's still an improvement for them.

The exact renaming is as follows:

- rustc_expand/src/mut_visit/tests.rs -> rustc_parse/src/parser/mut_visit/tests.rs
- rustc_expand/src/tokenstream/tests.rs -> rustc_parse/src/parser/tokenstream/tests.rs
- rustc_expand/src/tests.rs + rustc_expand/src/parse/tests.rs ->
  compiler/rustc_parse/src/parser/tests.rs

The latter two test files are combined because there's no need for them
to be separate, and having a `rustc_parse::parser::parse` module would
be weird. This also means some `pub(crate)`s can be removed.

compiler/rustc_expand/src/lib.rs

@@ -34,24 +34,4 @@ pub mod module;
 #[allow(rustc::untranslatable_diagnostic)]
 pub mod proc_macro;
 
-// HACK(Centril, #64197): These shouldn't really be here.
-// Rather, they should be with their respective modules which are defined in other crates.
-// However, since for now constructing a `ParseSess` sorta requires `config` from this crate,
-// these tests will need to live here in the interim.
-
-#[cfg(test)]
-mod tests;
-#[cfg(test)]
-mod parse {
-    mod tests;
-}
-#[cfg(test)]
-mod tokenstream {
-    mod tests;
-}
-#[cfg(test)]
-mod mut_visit {
-    mod tests;
-}
-
 rustc_fluent_macro::fluent_messages! { "../messages.ftl" }