tests: remove dummy test

I was experimenting and this test obviously found a very good hiding
place.
Make the case in dictionary lookup configurable

Essentially, this allows you to complete long words at the beginning of a
sentence.  The default is still to match only case-sensitively because
uppercase names would come first which is awful when completing short
leading words.

A workaround is to copy the dictionary, resort, and set the new variable to one:

    b:LocalCompleteWantIgnoreCaseDict
    g:localcomplete#WantIgnoreCaseDict
Introduce an inactive function that applies &infercase to matches

Vim's completions adapt the matches to the case of the leading word if
&infercase and &ignorecase are set.
Apply &infercase to all three completions
tests: test infercase behavior in system tests

The test for localMatches() already had that included in one position.
De-duplicate casematch flag getters into one function
tests: Rename the mock parameter to want_ignorecase_local

This differentiates it more clearly from the dict version and also from
the test-case helper function parameters.
README: Add notes about the new case features