Eglot: no more tests based on Pylsp (bug#62694)

Eglot: no more tests based on Pylsp (bug#62694)

The functionality under test in eglot.el is exactly the same, but use
the clangd server only, as that is used in more tests, and it is much
easier to check if it misbehaves or not.

Tests pass with clangd version 15.

* test/lisp/progmodes/eglot-tests.el (python): Don't require it.
(eglot--call-with-fixture): Simplify.
(eglot--wait-for-clangd): New helper.
(eglot-test-basic-completions)
(eglot-test-non-unique-completions, eglot-test-basic-xref)
(eglot-test-snippet-completions)
(eglot-test-snippet-completions-with-company)
(eglot-test-eldoc-after-completions, eglot-test-multiline-eldoc):
Use clangd, not pylsp.
(eglot-test-formatting): Renamed from
eglot-test-python-autopep-formatting.
(eglot-test-python-yapf-formatting): Remove.