tests/fixedbugs: regression test for issue 100 (#129)

Adds a regression test based on klauspost/compress#186. This necessitated some related changes:

* Mark "RET" as a terminal instruction
* printer refactor to maintain compatibility with asmfmt
* Tweaks to other regression tests to ensure they are run correctly in CI

Updates #100 #65 #8

internal/gen/ctors.go

@@ -98,6 +98,10 @@ func construct(i inst.Instruction, f inst.Form, s signature) string {
 	}
 
 	// Branch variables.
+	if i.IsTerminal() {
+		fmt.Fprintf(buf, "\tIsTerminal: true,\n")
+	}
+
 	if i.IsBranch() {
 		fmt.Fprintf(buf, "\tIsBranch: true,\n")
 		fmt.Fprintf(buf, "\tIsConditional: %#v,\n", i.IsConditionalBranch())