Make testsuite suitable for interactive use

Test suite can now be loaded with M-x eval-buffer in testsuite.el.
Useful for quick testing of individual functions.
Fix error message on checker timeout
Fix tagline
[Fix #80] Defer syntax checks for invisible buffers
Remove obsolete functions

[ci skip]
Test for validity before describing a checker
Navigate to checker definitions
Properly plug Flycheck into find-func
Define help button after help-mode is loaded

Fixes load error about unknown button type `help-xref`.
Fix spelling
Document required PHP CodeSniffer version [#78]