Give test-completion's PREDICATE full alist entry

Give test-completion's PREDICATE full alist entry

Since 2016-06-26 "Fix test-completion with completion-regexp-list", when
calling test-completion with an alist collection, the predicate was
recieving the string value instead of the alist entry (Bug#24966).

* src/minibuf.c (Ftest_completion): Don't modify the found element, just
test STRING against `completion-regexp-list'.
* test/src/minibuf-tests.el: New tests for `try-completion',
`all-completions', and `test-completion'.