test: add test_expect_equal_file_nonempty

A common bug in tests is that the code used to generate the EXPECTED
file fails, generating no output. When the code generating the OUTPUT
file fails in the same way, the test passes, even though there is a
failure being hidden. Add a new test function that guards against
this.

test/test-lib.sh

@@ -434,6 +434,26 @@ test_expect_equal_file () {
     test_diff_file_ "$1" "$2"
 }
 
+# Like test_expect_equal, but takes two filenames. Fails if either is empty
+test_expect_equal_file_nonempty () {
+    exec 1>&6 2>&7		# Restore stdout and stderr
+    if [ -z "$inside_subtest" ]; then
+	error "bug in the test script: test_expect_equal_file_nonempty without test_begin_subtest"
+    fi
+    inside_subtest=
+    test "$#" = 2 ||
+	error "bug in the test script: not 2 parameters to test_expect_equal_file_nonempty"
+
+    for file in "$1" "$2"; do
+	if [ ! -s "$file" ]; then
+	    test_failure_ "Missing or zero length file: $file"
+	    return $?
+	fi
+    done
+
+    test_diff_file_ "$1" "$2"
+}
+
 # Like test_expect_equal, but arguments are JSON expressions to be
 # canonicalized before diff'ing.  If an argument cannot be parsed, it
 # is used unchanged so that there's something to diff against.