LIB-26: add a simple test with a disc

This basically replicates the testing already done in python-discid.

I implemented several helper functions. That should pay off when adding
more tests and using these in the build test suite.
LIB-26: use testing functions in both tests
use noinst_HEADERS for all internal headers

Not really sure why these have been in libdiscid_la_SOURCES before
and I hope this doesn't break somehow.
LIB-26: run test_disc, but skip it without disc

Autotools supports a return value of 77 to skip the test
so we do that.
Cmake doesn't support testind directly anyways so we have
to catch the return type 77 manually.

return values other than 0 and 77 should lead to errors of course.
LIB-26: split tests to core, put and read

One program is one test for autotools so we should split that up
to give reasonable output.
FAILED: test_discid
wouldn't give much information on what failed.
change test_read to the new testing schema

This means we can get rid of the msg parameter and assert_true.
LIB-26: test full read (with MCN + ISRCS)

We can only check that nothing invalid happens.
Discs might not include any MCN or ISRCS, even if libdiscid works
correctly.
LIB-26: test error_msg, features and feature_list

Error message should be available when something failed and otherwise
empty.
Features are tested for consistency with the feature list and the actual
output on read.
add tests for get_default_device

We also anticipate tests for empty objects,
which wouldn't work right now so it is disabled.
check read/put idempotence

When using the output of a read() as input to put(), we should get the
same result.
add missing include

Warning in Windows, already included in other platform code.
fix printf warning

Interestingly enough, only Mac OS X complained about
the first argument of printf not being a constant string
and not including formatting placeholders.
Merge branch 'more_tests'

see pull-request #26