Use a global test seed for all unit tests

• Provide an inline helper function to request a seed stream for a given key.
• Construct unique seeds by combining the global test seed with key-specific hash.
• Update all unit tests to use the new seed stream mechanism.
• Update fn_generate_* functions in MatMulMethod to take in a SeedFeed reference to delay seed generation until after global test seed has been set.
• Add a listener to test/nextgen/main.cpp that prints the test seed at end of every test program run.

Signed-off-by: Jakub Sujak jakub.sujak@arm.com Signed-off-by: James Gross james.gross@arm.com