chore: fix use of deprecated methods in tests/test_language_server.py

[RossBencina]

Description (e.g. "Related to ...", etc.)

Fixes the following two DeprecationWarnings:

tests/test_language_server.py::test_bf_text_document_did_open
  C:\_\pygls\tests\test_language_server.py:85: DeprecationWarning: 'workspace.documents' has been deprecated, use 'workspace.text_documents' instead
    assert len(server.lsp.workspace.documents) == 1

tests/test_language_server.py::test_bf_text_document_did_open
  C:\_\pygls\tests\test_language_server.py:87: DeprecationWarning: 'workspace.get_document' has been deprecated, use 'workspace.get_text_document' instead
    document = server.workspace.get_document(__file__)

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly

[tombh]

Thank you again 🙇