cmd/mbf: do not abort cache acquisition during testing

This can sometimes fire during testing due to how short the test is.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/mbf/cache.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"os"
 	"path/filepath"
+	"testing"
 
 	"hakurei.app/check"
 	"hakurei.app/internal/pkg"
@@ -53,6 +54,9 @@ func (cache *cache) open() (err error) {
 	go func() {
 		select {
 		case <-cache.ctx.Done():
+			if testing.Testing() {
+				return
+			}
 			os.Exit(2)
 
 		case <-done: