When copying test binary, delete first (hashicorp#19331)

For plugin tests, we copy the test binary. On macOS, if the
destination binary already exists, then copying over it will result
in an invalid signature.

The easiest workaround is to delete the file before copying.

vault/testing.go

@@ -526,6 +526,9 @@ func TestAddTestPlugin(t testing.T, c *Core, name string, pluginType consts.Plug
 
 		// Copy over the file to the temp dir
 		dst := filepath.Join(tempDir, fileName)
+
+		// delete the file first to avoid notary failures in macOS
+		_ = os.Remove(dst) // ignore error
 		out, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, fi.Mode())
 		if err != nil {
 			t.Fatal(err)