Kill garbage
Fix headline comment start
Heavily refactor the tests

Split flycheck-should-checker into different functions, with clear
responsibilities.

Also run checkers with the mostly complete flycheck-checkers, to ensure
the proper ordering and the absence of conflicts in this variable.

Un-breaks test with only a single expected error.
[Fix #65] Do not collapse whitespace

Keep internal formatting of diagnostics intact, which may be very
relevant to the understand of the diagnostics.
Fix syntax error in phpcs test case
Fix checker chaining with predicates

We must not retrieve the next checker before having assigned
`flycheck-current-errors`, because that obviously breaks predicate
evaluation.