Fix a lot of linting issues

After we introduced the linter we haven't really touched the old code.
This commit now fixes all linting issue that exist right now.

util/util_test.go

@@ -2,6 +2,7 @@ package util
 
 import (
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	"gitlab.hpi.de/codeocean/codemoon/poseidon/tests"
 	"testing"
 )
@@ -10,8 +11,9 @@ func TestNullReaderDoesNotReturnImmediately(t *testing.T) {
 	reader := &NullReader{}
 	readerReturned := make(chan bool)
 	go func() {
-		p := make([]byte, 5)
-		_, _ = reader.Read(p)
+		p := make([]byte, 0, 5)
+		_, err := reader.Read(p)
+		require.NoError(t, err)
 		close(readerReturned)
 	}()
 	assert.False(t, tests.ChannelReceivesSomething(readerReturned, tests.ShortTimeout))