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_test.go

@@ -1,7 +1,6 @@
 package main
 
 import (
-	"context"
 	"log"
 	"os"
 	"testing"
@@ -14,7 +13,7 @@ func TestCache(t *testing.T) {
 	t.Parallel()
 
 	cm := cache{
-		ctx:  context.Background(),
+		ctx:  t.Context(),
 		msg:  message.New(log.New(os.Stderr, "check: ", 0)),
 		base: t.TempDir(),