Rollup merge of #146450 - lolbinarycat:bootstrap-test-js, r=jieyouxu

bootstrap: rustdoc-js tests can now be filtered by js files

Before, a command like `./x test tests/rustdoc-js/path-ordering.js` would succeed, but run no tests, since the names of the tests are based on the `.rs` file.  This is a bit confusing, as the `rustdoc-js-std` test suite only has `.js` files, and thus those are the files you filter on.  Now, `./x test tests/rustdoc-js/path-ordering.js` will be treated as an alias for `./x test tests/rustdoc-js/path-ordering.rs`.  This is fairly simple as each `rustdoc-js` test has 2 files, 1 js file and one rust file, each with an identical base filename, so all we need to do is swap the extension.

r? `@Kobzol`

Author: jhpratt