libunwind: disable installation of tests dir

libunwind: disable installation of tests dir

fix the issue that:
| Error: Transaction test error:
|   file /usr/libexec/libunwind/check-namespace.sh conflicts between attempted installs of libunwind-1.8.1-r0.core2_64 and lib32-libunwind-1.8.1-r0.core2_32
|   file /usr/libexec/libunwind/test-runner conflicts between attempted installs of libunwind-1.8.1-r0.core2_64 and lib32-libunwind-1.8.1-r0.core2_32

By default, test suite from srcdir/tests/* will be installed to /usr/libexec/libunwind, here pass --disable-tests to not install test suite.
Test suite can be added to libunwind-ptest in the future if needed

Signed-off-by: Liu Yiding <liuyd.fnst@fujitsu.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>