Fix: Conditionally disable test requiring shared libs

When building lttng-tools with shared library support explicitly
disabled (e.g.: ./configure --disable-shared), libtool fail with a fatal
error:

Making all in demo
  CC       tp.lo
  CC       tp2.lo
  CC       tp3.lo
  CC       demo.o
  CCLD     demo
  CCLD     liblttng-ust-provider-ust-tests-demo.la
  CCLD     liblttng-ust-provider-ust-tests-demo3.la
libtool: link: can not build a shared library
libtool: link: See the libtool documentation for more information.
libtool: link: Fatal configuration error.

The build should not fail because some tests require explicit shared
library support, instead they should be skipped. This can be
accomplished by detecting builds with no shared libs support with the
NO_SHARED Automake variable.

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>