respect completion prefix when completing optional arguments

[awilliambauer]

For completions on labels that the LSP gets from merlin, take into account whether the prefix being completed starts with ~ or ?.

Merlin appears to always returns the full set of labeled/optional arguments when asked for completions. The LSP forwards these on, leaving it to the client to filter them. The problem is that merlin supplies the optional argument completions with a leading ? regardless of the prefix the user typed. This causes the client to either filter them out (VSCode) or offer completions that might flip an optional argument being passed with ~ to one passed with ? (Emacs).

This PR changes the label completions that start with ? to start with ~ when the prefix being completed starts with ~. It also adds a test case and does a bit of refactoring of test helper functions.

[coveralls]

Pull Request Test Coverage Report for Build 4230

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 21.181%

---

Totals
Change from base Build 4225:	0.03%
Covered Lines:	5145
Relevant Lines:	24291

---

💛 - Coveralls

[rgrinberg]

There were some async leftovers I had to tweak. LGTM now.