tests: autogenerated test to confirm attribute strings

Test helps verify our attribute types match the macros in textflag.h.
Discovered https://golang.org/issue/29487 in the process.

Updates #2

tests/textflag/attrtest_test.go

@@ -0,0 +1,13 @@
+package textflag
+
+import "testing"
+
+//go:generate go run make_attrtest.go -output zattrtest.s -seed 42 -num 256
+
+func AttrTest() bool
+
+func TestAttributes(t *testing.T) {
+	if !AttrTest() {
+		t.Fail()
+	}
+}