Rollup merge of #132828 - est31:let_chains_parsing_tests, r=compiler-errors

Additional tests to ensure let is rejected during parsing

In the original stabilization PR, @ `compiler-errors` has [pointed out](rust-lang/rust#94927 (comment)) that #97295 wasn't enough to address the concerns about having `let` in expressions being rejected at parsing time, instead of later.

Thankfully, since then the situation has been greatly improved by #115677. This PR adds some additional tests to `disallowed-positions.rs`, and adds two additional revisions to the "normal" case which is now given the `feature` name:

* `no_feature`: Added to incorporate `disallowed-positions-without-feature-gate.rs` into the file, reducing duplication.
* `nothing`: like feature, but all functions are cfg'd out. Ensures that the errors are really emitted during parsing.

cc tracking issue #53667

Author: workingjubilee