fuzz: remove UNITTEST dependency

fuzz: remove UNITTEST dependency

Expose UTH flow builder to new 'FUZZ' define as well. Move UTHbufferToFile
as well and rename it to a more generic 'TestHelperBufferToFile'.

This way UNITTESTS can be disabled. This leads to smaller code size
and more realistic testing as in some parts of the code things
behave slightly differently when UNITTESTS are enabled.