build: fix 'make check' linkage with dtrace

Building on Linux with dtrace enabled was failing 'make check':

  CCLD   nodeinfotest
../src/.libs/libvirt_test.a(libvirt_net_rpc_client_la-virnetclient.o): In function `virNetClientNew':
/home/remote/eblake/libvirt/src/rpc/virnetclient.c:162: undefined reference to `libvirt_rpc_client_new_semaphore'

On looking further, I see some earlier warnings emitted from libtool:

*** Warning: Linking the shared library libvirt.la against the non-libtool
*** objects  probes.o is not portable!

Since src/probes.o is only built on Linux, and even then, only when
dtrace is enabled, this failure does not affect other platforms, and
despite libtool warning that it is not generally portable, it is not
a problem for our use-case in libvirt.la.  But it turns out that while
libtool is willing to jam raw .o files into an installed shared
library (libvirt.la becomes libvirt.so), it is NOT willing to jam
the same .o file into the convenience library libvirt_test.la.
Perhaps this is a bug in libtool, but even if we get libtool fixed,
libvirt must continue to build on platforms with older libtool.  So,
the fix is the same as we are already using for the libvirt_lxc
executable - don't rely on the .o file being in the convenience
library, but instead use LDADD to pull it in directly.

* tests/Makefile.am (PROBES_O): New macro.
(LDADDS): Use it to fix link errors.

tests/Makefile.am

@@ -29,6 +29,11 @@ INCLUDES += \
 	-DTEST_DRIVER_DIR=\"$(top_builddir)/src/.libs\"
 endif
 
+PROBES_O =
+if WITH_DTRACE
+PROBES_O += ../src/probes.o
+endif
+
 LDADDS = \
 	$(STATIC_BINARIES) \
 	$(LIBXML_LIBS) \
@@ -39,6 +44,7 @@ LDADDS = \
         $(YAJL_LIBS) \
         $(WARN_CFLAGS) \
 	../src/libvirt_test.la \
+	$(PROBES_O) \
 	../gnulib/lib/libgnu.la \
 	$(LIBSOCKET) \
         $(COVERAGE_LDFLAGS)