tests/attrtest: move decl to non-test file

Solves the asmdecl error:

  [amd64] AttrTest: function AttrTest missing Go declaration

Updates #24

tests/textflag/attrtest.go

@@ -0,0 +1,6 @@
+// Package textflag tests that avo attribute constants agree with textflag.h.
+package textflag
+
+//go:generate go run make_attrtest.go -output zattrtest.s -seed 42 -num 256
+
+func attrtest() bool