Test preview config

Support for preview config option was introduced in commit
4b80007.

This adds a test that the config option is parsed by python-lsp-black.

Author: haplo

tests/fixtures/config/pyproject.toml

@@ -4,3 +4,4 @@ line-length = 20
 pyi = true
 skip-magic-trailing-comma = true
 skip-string-normalization = true
+preview = true

tests/test_plugin.py

@@ -235,7 +235,7 @@ def test_load_config(config):
         "fast": True,
         "skip_magic_trailing_comma": True,
         "skip_string_normalization": True,
-        "preview": False,
+        "preview": True,
     }