Test that the help's examples read no data and list the exit codes, a data path that is no directory says so in one shape, a choice suggests a record of one, a JSON syntax error names its line and column, and FakeStruct says no data is loaded

internal/jsonvalue/jsonvalue_test.go

@@ -0,0 +1,15 @@
+package jsonvalue
+
+import "testing"
+
+func TestDecodeNamesTheLineAndColumnOfASyntaxError(t *testing.T) {
+	for in, want := range map[string]string{
+		"{\"a\": 1,}":                  "line 1, column 9: invalid character '}' looking for beginning of object key string",
+		"{\n  \"a\": 1\n  \"b\": 2\n}": "line 3, column 3: invalid character '\"' after object key:value pair",
+		"":                             "line 1, column 1: unexpected end of JSON input",
+	} {
+		if _, err := Decode([]byte(in)); err == nil || err.Error() != want {
+			t.Errorf("Decode(%q) = %v, want %q", in, err, want)
+		}
+	}
+}