`x test` doesn't work for absolute paths

[oriongonza]

❯ x test    /home/ardi/repos/rust/tests/ui/impl-trait/precise-capturing/unexpected-token.rs
Building bootstrap
   Compiling build_helper v0.1.0 (/home/ardi/repos/rust/src/tools/build_helper)
   Compiling bootstrap v0.0.0 (/home/ardi/repos/rust/src/bootstrap)
    Finished `dev` profile [unoptimized] target(s) in 7.20s
Updating submodule src/tools/cargo
fatal: ref HEAD is not a symbolic ref
Submodule path 'src/tools/cargo': checked out '3ed207e416fb2f678a40cc79c02dcf4f936a21ce'
ERROR: no `test` rules matched ["/home/ardi/repos/rust/tests/ui/impl-trait/precise-capturing/unexpected-token.rs"]
HELP: run `x.py test --help --verbose` to show a list of available paths
NOTE: if you are adding a new Step to bootstrap itself, make sure you register it with `describe!`
Build completed unsuccessfully in 0:00:08