tests: tweak built_programs definition

* tests/Makefile.am (built_programs): Adjust to work around what
may be a problem due to interaction between Solaris 10's /bin/sh
and an old version of GNU make. Reported by Dagobert Michelsen
in https https://bugs.gnu.org/24137.

tests/Makefile.am

@@ -79,7 +79,11 @@ LOG_COMPILER= $(SHELL)
 
 built_programs =							\
   echo 'spy:;@echo $$(PROGRAMS)'					\
-    | (cd ../src && MAKEFLAGS= $(MAKE) -s -f Makefile -f - spy)		\
-    | tr ' ' '\n' | sed '/^$$/d; s,$(EXEEXT)$$,,' | sort -u
+    | { (cd ../src && MAKEFLAGS= $(MAKE) -s -f Makefile -f - spy)	\
+          | tr ' ' '\n'							\
+          | sed '/^$$/d; s,$(EXEEXT)$$,,'				\
+          | sort -u							\
+          | tr '\n' ' '; echo; }					\
+    | sed 's/ $$//'
 
 VERBOSE = yes