What causes 'no such file or directory' after adding a test case and running go test?

The problem

After adding another test function to an existing test file running go test -v ./... fails due to several no such file or directory build errors after adding another test case. The error messages are seemingly unrelated to the changes, however.

The added test case can be found in the relevant code section at the bottom.

The error messages are:

open /tmp/go-build842273301/b118/vet.cfg: no such file or directory
open /tmp/go-build842273301/b155/vet.cfg: no such file or directory
# tornadowarnung.xyz/riotwatch/riot/static
vet: in tornadowarnung.xyz/riotwatch/riot/static, can't import facts for package "encoding/json": open $WORK/b036/vet.out: no such file or directory
# tornadowarnung.xyz/riotwatch/web/server/endpoint/static
vet: open $WORK/b121/vet.cfg: no such file or directory
open /tmp/go-build842273301/b115/vet.cfg: no such file or directory
open /tmp/go-build842273301/b001/vet.cfg: no such file or directory
# tornadowarnung.xyz/riotwatch/web/server
vet: open $WORK/b152/vet.cfg: no such file or directory
# tornadowarnung.xyz/riotwatch/web/server/endpoint/static
vet: open $WORK/b159/vet.cfg: no such file or directory

Because of that, some packages show their build failed:

FAIL    tornadowarnung.xyz/riotwatch/riot/static [build failed]
FAIL    tornadowarnung.xyz/riotwatch/web/server [build failed]
FAIL    tornadowarnung.xyz/riotwatch/web/server/endpoint [build failed]
FAIL    tornadowarnung.xyz/riotwatch/web/server/endpoint/static [build failed]

Relevant code

func TestLoader_ProfileIcon(t *testing.T) {
    tempDir := os.TempDir()
    l := Loader{
        profileIconPath: tempDir,
    }
    defer os.RemoveAll(tempDir)

    t.Run("returns expected content", func(t *testing.T) {
        want := bytes.NewBufferString("image data")
        fileName := "123456"
        if err := createTestFile(t, tempDir, fileName, want); err != nil {
            t.Fatal(err)
        }

        got, err := l.ProfileIcon(123456)
        if err != nil {
            t.Error(err)
        }

        if !reflect.DeepEqual(got, want) {
            t.Errorf("got %v, want %v", got, want)
        }
    })

    t.Run("does not panic on missing file", func(t *testing.T) {
        res, err := l.ProfileIcon(-1)

        if err == nil {
            t.Errorf("Expected an error but got error %v and result %v", nil, res)
        }
    })
}

func createTestFile(t *testing.T, tempDir string, fileName string, content *bytes.Buffer) error {
    t.Helper()
    f, err := os.Create(path2.Join(tempDir, fmt.Sprintf("%v.png", fileName)))
    if err != nil {
        return err
    }
    _, err = f.Write(content.Bytes())
    if err != nil {
        return err
    }
    return nil
}

Reproducing the error is difficult

On my Ubuntu machine having go 1.15 installed the error only occurs sometimes when I'm cloning the repository again or when I'm cleaning the test cache.

When running the image used in the Gitlab job golang:alpine locally and running the same commands I cannot reproduce this error every time. Sometimes it occurs but most of the time it doesn't.

What I've tried

I have tried to switch between go versions 1.13, 1.14, and 1.15 but every version yields the same result.

Switching to any other images like golang:latest, golang:1.14 or golang:1.13 doesn't help either.

I've tried googling for the error that occurs but I haven't found any results that are relevant or contain any solution I haven't already tried.

Reverting said commit will make the tests pass again. I've also reverted the commit and slowly tried to introduce the changes again manually. This makes the problems occur again.