Remove illegal attribute prefixes in testbenches.
A number of testbenches contain code on the form assert function_call(parameters)'length = 0; The purpose is to verify that the function returns a null vector. However, the length attribute can only be applied to array objects and a function call returning an array is not an object but an expression. This is strictly enforced in the latest versions of GHDL. This commit updates the testbenches violating this rule.