pass: some basic allocator test cases

pass/alloc.go

@@ -64,11 +64,15 @@ func (a *Allocator) Add(r reg.Register) {
 }
 
 func (a *Allocator) Allocate() (reg.Allocation, error) {
-	for a.remaining() > 0 {
+	for {
 		if err := a.update(); err != nil {
 			return nil, err
 		}
 
+		if a.remaining() == 0 {
+			break
+		}
+
 		v := a.mostrestricted()
 		if err := a.alloc(v); err != nil {
 			return nil, err