[RFC] tests: make read_text() work with directories (#4155)

This PR is just a showcase to see how we like it. I've converted a few
tests to it, but there will be much more to come.

```
assert (tmp_dir / "file") == "contents"
assert (tmp_dir / "dir").read_text() == {"subdir": {"file": "hello"}}
```

This approach is nicer because it allows us to use a declaratory
asserts instead of relying on fs files. The problem becomes much more
apparent once you get into remote workspaces, where we `trees_equal`
doesn't work.

tests/utils/__init__.py

@@ -1,6 +1,5 @@
 import os
 from contextlib import contextmanager
-from filecmp import dircmp
 
 from dvc.scm import Git
 
@@ -20,15 +19,5 @@ def cd(newdir):
         os.chdir(prevdir)
 
 
-def trees_equal(dir_path_1, dir_path_2):
-
-    comparison = dircmp(dir_path_1, dir_path_2)
-
-    assert set(comparison.left_only) == set(comparison.right_only) == set()
-
-    for d in comparison.common_dirs:
-        trees_equal(os.path.join(dir_path_1, d), os.path.join(dir_path_2, d))
-
-
 def to_posixpath(path):
     return path.replace("\\", "/")