perf: make manifest flags testing feature compile itself out when in built in production mode

See the change at https://github.com/MetaMask/metamask-extension/pull/26588

Start by reviewing the change referenced at https://github.com/MetaMask/metamask-extension/pull/26588 and trace how manifest flags are handled in production builds. Confirm which entry points and build configuration are involved, then verify that the testing feature is compiled out when production mode is built.

Written by the indexing model from the issue text.