Refactored testbench code

Created by: silabs-hfegran

• Moved iss-initalization to uvm_reset phase
• Moved ref_init to an interface instead of inside imperas_dv wrapper module to easily access function with uvm
• Fixed issue with ISS in regressions
• Added iss dummy package to support building with and without iss installed and remove all compile time ifdef USE_ISS
• Split imperas manifest file in two, to resolve dependencies with initialization in uvm-code

Future work: change testplusarg("USE_ISS") to depend on uvm configuration setting.