in case the real paths into the libstd/libcore are located inside the
the build directory, maybe because it's coming from an extracted dist
component in the build dir (cc opt-dist)

This is done to cover the path of the test it-self as it may not live
on the same root directory as {{rust-src-base}}, which can be the case
if {{rust-src-base}} is coming from a extracted dist build (cc opt-dist)