Move `tmp` test directory.

The `tmp` directory added in rust-lang#9375 was placed within the profile directory (such as `target/debug/tmp` or `target/release/tmp`).  This causes problems for any cargo target (binary, test, etc.) with the name `tmp` as there is a name collision.  This PR attempts to address that by moving the `tmp` directory to the root of the target directory (`target/tmp`), and reserving the profile name "tmp".

Fixes rust-lang#9783

Temporarily disable extern-html-root-url test.

A change in rust-lang/rust#82776 broke this test,
so disabling for now until it is figured out how things are going to work.