Add --disable-tests option (#475)

Add --disable-tests to allow the user to disable tests. As a
side-effect, this will avoid the following build failure when check is
found:

libstat_wrapper.c:11:10: fatal error: gnu/lib-names.h: No such file or directory

   11 | #include <gnu/lib-names.h>
      |          ^~~~~~~~~~~~~~~~~

This build failure is raised since version 2.0.5 and
https://github.com/ClusterLabs/libqb/commit/78df90b180740712d0c90b6d982b78241cc99d72

Fixes:

• http://autobuild.buildroot.org/results/450cfc36d4fd6dc71c138bec45f05b5a2d92a08d

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>