test: add empty/non-empty input checkers

When running a pipeline on a command's output, a simple check is making
sure the pipeline returned something empty or not.

This is the goal of those two implementations, basically either the
input is empty as expected, or it errors, and the reverse.

test/pipe_checker_test.go

@@ -47,6 +47,28 @@ func (ct CustomTester) Check(input string) error {
 	return ct(input)
 }
 
+// NonEmptyInput errors if the result from the pipeline was empty
+func NonEmptyInput() Tester {
+	return CustomTester(func(in string) error {
+		in = strings.TrimSpace(in)
+		if in != "" {
+			return fmt.Errorf("input is not empty, expected it to be: %s", in)
+		}
+		return nil
+	})
+}
+
+// EmptyInput errors if the result from the pipeline was not empty
+func EmptyInput() Tester {
+	return CustomTester(func(in string) error {
+		in = strings.TrimSpace(in)
+		if in == "" {
+			return fmt.Errorf("input is empty, expected it not to")
+		}
+		return nil
+	})
+}
+
 // PipeChecker is a kind of checker that essentially lets users write mini
 // gadgets that pipe inputs together, and compose those to end as a true/false
 // statement, which translates to an error.