Test: Prevent use of transport client when testing changed transports

This makes sure, that the transport client is not used, as it would use
a different than the configured local transport mechanism in this test.
[TEST] UnicastBackwardsCompatibilityTest should not copy internal node settings to external nodes

Recent test failures triggered by #7289 were caused by this, simply because internal node settings (transport type key) that are not supported by the external older nodes were copied to them by mistake.
Internal: adjusted internal requests visibility from public to package private