Use installed package to execute buildonly test.

Index: conformance/interfaces/sigrelse/3-4.sh
--- conformance/interfaces/sigrelse/3-4.sh.orig
+++ conformance/interfaces/sigrelse/3-4.sh
@@ -9,4 +9,4 @@
 #   Test various methods of adding invalid signals to sigrelse().
 # 
 
-conformance/interfaces/sigrelse/3-core-buildonly.test 4
+${TRUEPREFIX}/libexec/posixtestsuite/conformance/interfaces/sigrelse/3-core-buildonly.test 4
