tests: fail the build if symlinking tests/ostree fails

Signed-off-by: Simon McVittie <smcv@debian.org>

Closes: #367
Approved by: cgwalters

Makefile-tests.am

@@ -245,7 +245,8 @@ ALL_LOCAL_RULES += tests/libreaddir-rand.so
 CLEANFILES += tests/libreaddir-rand.so tests/ostree-symlink-stamp tests/ostree
 
 tests/ostree-symlink-stamp: Makefile
-	@real_bin=`cd $(top_builddir) && ./libtool --mode=execute echo ostree`; \
+	@set -e; \
+	real_bin=`cd $(top_builddir) && ./libtool --mode=execute echo ostree`; \
 	ln -sf "$${real_bin}" tests/ostree; \
 	touch $@