More robust import tests

[arduano]

More robust import tests

Everything I've done in previous PRs fixed issues with these tests. They test importing both using paths and strings, importing from the same folder, from a parent folder, from a child folder.

[urbas]

Looks like imports with relative paths as strings is not supported in nix:

% nix eval -f child-folder-import.nix     
error:
       … while evaluating the attribute 'dataString'

         at /home/mat/git/rix/src/tests/import_tests/child-folder-import.nix:3:3:

            2|   dataPath = (builtins.import ./nested/basic.nix).data;
            3|   dataString = (builtins.import "./nested/basic.nix").data;
             |   ^
            4| }

       … while calling the 'import' builtin

         at /home/mat/git/rix/src/tests/import_tests/child-folder-import.nix:3:17:

            2|   dataPath = (builtins.import ./nested/basic.nix).data;
            3|   dataString = (builtins.import "./nested/basic.nix").data;
             |                 ^
            4| }

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: string './nested/basic.nix' doesn't represent an absolute path

[urbas]

Looks great. The only thing needed is to convert the "importing with relative paths as strings" tests. These tests should produce an import error, so could you please assert that the error is thrown?