Test framework enhancements, fix the check_exports test

* Use "printf" instead of "echo -n" as it's more portable
* Treat a test as skipped if it exits with exit status of 77
* Skip the check_exports test if "nm -D" doesn't work

test/suites/api/check-exports

@@ -89,7 +89,10 @@ EOF
 
 SOFILE="../src/.libs/libjansson.so"
 
-nm -D $SOFILE | grep ' T ' | cut -d' ' -f3 | sort >$test_log/output
+nm -D $SOFILE >/dev/null >$test_log/symbols 2>/dev/null \
+    || exit 77  # Skip if "nm -D" doesn't seem to work
+
+grep ' T ' $test_log/symbols | cut -d' ' -f3 | sort >$test_log/output
 
 if ! cmp -s $test_log/exports $test_log/output; then
     diff -u $test_log/exports $test_log/output >&2