Fix eglot-completion-at-point for multiple matches

Fix eglot-completion-at-point for multiple matches

The test-completion case shouldn't return t when there are multiple
matches.  Similarly, the try-completion should return t only if the
match is exact.  See (info "(elisp)Programmed Completion").

* eglot.el (eglot-completion-at-point): Instead of testing
memberships, use test-completion and try-completion suggested
by (info "(elisp)Programmed Completion").

* eglot-tests.el (non-unique-completions): Add new test.

Co-authored-by: João Távora <joaotavora@gmail.com>
GitHub-reference: fix https://github.com/joaotavora/eglot/issues/365