bootstrap: `core::config::tests::detect_src_and_out` test fails if checkout is not under `C` drive

[jieyouxu]

On native Windows:

---- core::config::tests::detect_src_and_out stdout ----
thread 'core::config::tests::detect_src_and_out' panicked at src\core\config\tests.rs:72:13:
assertion `left == right` failed
  left: "E:\\tmp"
 right: "C:\\tmp"

[ChrisDenton]

This looks sus:

test(parse("build.build-dir = \"/tmp\""), build_dir);

That's a /tmp which is an absolute Unix path. On Windows it will be interpreted relative to the root of the current directory. For example, if your current directory is R:\path\to\rust then it'll be R:\tmp.

(Also it'd be nice to use raw strings here because the \"\" is confusing in the context of paths)