Remove testers documentation

Having our "end-user" documentation point at the manual pages of the
testers only causes confusion.  (It confuses me, so it must also confuse
users.)  Because the tester binaries are internal at this point and I
have plans to kill them, just remove their documentation for simplicity.

While doing this, assimilate any useful remains into our other
documentation.  In particular, the only details that are worth keeping
are the fact that we now have support for TAP test programs and the list
of supported metadata properties.  These should all have gone into
kyuafile(5) in the first place.
Add a new kyua-test-isolation(7) page

This new page describes the environment in which each test case or test
program is run.  The contents come from the existing kyua-atf-interface(7)
page.
Remove the kyua-*-interface(7) pages

These pages are quite useless and only add confusion.  The definition of
each test interface belongs to the provider of such interface.

For example: ATF provides the atf(7) manual page (and supporting contents
such as atf-test-case(4) and atf-test-program(4)) and TAP is widely
documented on the Internet in various forms.  Kyua should just try to
support these upstream interfaces without re-explaining what they are.
Fix bad Xr entry
Add references to kyuafile(5)

While this manpage may have been discoverable on its own, it really
needs to be linked from many different places.  Add such links now.
Merge branch 'assimilate-testers-docs'

This removes all manual pages from the testers/ subdirectory by
assimilating their useful contents into other manpages, and also brings
in some miscellaneous documentation fixes.