Remove duplicate test CARv1 file

Remove the duplicate test CARv1 file that existed both in `testdata` and
`blockstore/testdata` in favour of the one in root.

Reflect change in tests.

Duplicity checked using `md5` digest:

```sh
$ md5 testdata/sample-v1.car
MD5 (testdata/sample-v1.car) = 14fcffc271e50bc56cfce744d462a9bd

$ md5 blockstore/testdata/test.car
MD5 (blockstore/testdata/test.car) = 14fcffc271e50bc56cfce744d462a9bd
```

v2/blockstore/readonly_test.go

@@ -37,11 +37,6 @@ func TestReadOnly(t *testing.T) {
 	}{
 		{
 			"OpenedWithCarV1",
-			"testdata/test.car",
-			newReaderFromV1File(t, "testdata/test.car"),
-		},
-		{
-			"OpenedWithAnotherCarV1",
 			"../testdata/sample-v1.car",
 			newReaderFromV1File(t, "../testdata/sample-v1.car"),
 		},

v2/blockstore/readwrite_test.go

@@ -52,7 +52,7 @@ func TestBlockstore(t *testing.T) {
 	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
 	defer cancel()
 
-	f, err := os.Open("testdata/test.car")
+	f, err := os.Open("../testdata/sample-v1.car")
 	require.NoError(t, err)
 	t.Cleanup(func() { assert.NoError(t, f.Close()) })
 	r, err := carv1.NewCarReader(f)
@@ -270,7 +270,7 @@ func (b bufferReaderAt) ReadAt(p []byte, off int64) (int, error) {
 }
 
 func TestBlockstoreNullPadding(t *testing.T) {
-	paddedV1, err := ioutil.ReadFile("testdata/test.car")
+	paddedV1, err := ioutil.ReadFile("../testdata/sample-v1.car")
 	require.NoError(t, err)
 
 	// A sample null-padded CARv1 file.
@@ -301,7 +301,7 @@ func TestBlockstoreNullPadding(t *testing.T) {
 }
 
 func TestBlockstoreResumption(t *testing.T) {
-	v1f, err := os.Open("testdata/test.car")
+	v1f, err := os.Open("../testdata/sample-v1.car")
 	require.NoError(t, err)
 	t.Cleanup(func() { assert.NoError(t, v1f.Close()) })
 	r, err := carv1.NewCarReader(v1f)