Some tests fail if using a non-default target directory

[Nemo157]

Problem
If you override the target directory and try running cargo's testsuite some of the tests fail to find their data.

Steps

> CARGO_TARGET_DIR=$(mktemp -d) cargo test -- standard_lib::simple_bin_std
[...]
     Running /tmp/tmp.7ILfdJhI9g/debug/deps/testsuite-f6e657c77e54be9a

running 1 test
test standard_lib::simple_bin_std ... FAILED

failures:

---- standard_lib::simple_bin_std stdout ----
running `/tmp/tmp.7ILfdJhI9g/debug/cargo build`
running `/tmp/tmp.7ILfdJhI9g/debug/cargo run -v -Zbuild-std --target x86_64-unknown-linux-gnu`
thread 'standard_lib::simple_bin_std' panicked at '
Expected: execs
    but: exited with exit code: 101
--- stdout

--- stderr
error: failed to read `/tmp/tests/testsuite/mock-std/library/std/Cargo.toml`

Caused by:
  No such file or directory (os error 2)
', crates/cargo-test-support/src/lib.rs:729:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    standard_lib::simple_bin_std

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 2088 filtered out; finished in 0.75s

Possible Solution(s)
Lookup files relative to the source files path, not the compiled binary.

[Nemo157]

@rustbot modify labels: +A-testing-cargo-itself