test: verify current remap rules on Linux

When `--remap-path-scope=object` is specified, user expect that there is
no local path embedded in final executables. Under `object` scope, the
current implementation only remap debug symbols if debug info is
splitted into its own file. In other words, when
`split-debuginfo=packed|unpacked` is set, rustc assumes there is no
embedded path in the final executable needing to be remapped.

However, this doesn't work on Linux. On Linux, the root `DW_AT_comp_dir`
of a compile unit seems to go into the binary binary executables.

This commit demonstrates the case, and hope there is a fix soon.