checktests: Make output TAP compliant

The output of checktests is now TAP compliant.

It can now even be run using Perl's prove utility like so:

  prove checktests

or with verbose output like so:

  prove -v checktests

In addition the use of grep has been removed so that only
a POSIX sh is required to run the tests.
README: Update test information

* Mention the checktests output is now TAP compliant
* Mention the ARM abihf clang bug