Rustdoc test suite: move more tests into `jump-to-def` dir

[lolbinarycat]

The following files would be better placed under rust/tests/rustdoc/jump-to-def:

• tests/rustdoc/jump-to-def-assoc-items.rs
• tests/rustdoc/jump-to-def-ice.rs
• tests/rustdoc/jump-to-def-ice-assoc-types.rs

[bikbov]

@rustbot claim

[bikbov]

@rustbot unclaim

[midsterx]

@rustbot claim