test: prevent segfault in imageverifier test

When VeriftImage returns an error, the first return value may be set to
nil (this is the case for timeouts). When that is the case, it makes no
point trying to access its fields later in the test. Let's use
testify/require instead of testify/assert here to ensure the test halts
upon failure.

Signed-off-by: Athos Ribeiro <athos.ribeiro@canonical.com>

pkg/imageverifier/bindir/bindir_test.go

@@ -175,7 +175,7 @@ func TestBinDirVerifyImage(t *testing.T) {
 		})
 
 		j, err := v.VerifyImage(ctx, "registry.example.com/image:abc", ocispec.Descriptor{})
-		assert.NoError(t, err)
+		require.NoError(t, err)
 		assert.True(t, j.OK, "expected OK, got not OK with reason: %v", j.Reason)
 		assert.Less(t, len(j.Reason), len(bins)*(outputLimitBytes+1024), "reason is: %v", j.Reason) // 1024 leaves margin for the formatting around the reason.
 	})