Also fixes retworkx Darwin build.
Fixes linker error for retworkx that required libiconv.
Error log:
error: linking with `/nix/store/47vpv5i10dwfg1cf5wca1k40f982g5fm-clang-wrapper-7.1.0/bin/cc` failed: exit code: 1
...
ld: library not found for -liconv
clang-7: error: linker command failed with exit code 1 (use -v to see invocation)
Also fix tests running on build, that didn't get converted when switched
to the new way of running Cargo builds for python packages.
