Test termination: Delegate decision to testbench instead of using explicit $finish.

[zchamski]

This PR replaces explicit use of $finish() in RVFI tracer with proper propagation of end-of-test information to the verification testbench:

• Instread of executing a $finish() statement the RFVI tracer now outputs the end-of-test event and the test exit code on new output end_of_test_o.
• The Verilator and VCS C++ testbenches were modified to detect the end-of-test event (write of 1'b1 into bit 0 of memory variable tohost) and to report the exit code of the test (bits [31:1] of the word written into tohost upon end-of-test). The Verilator testbench end-of-test messages were aligned on those of the VCS testbench.
• The UVM handling of the end-of-test is implemented in the companion core-v-verif PR UVM test termination: Use end-of-test output of RVFI tracer iso. $finish. core-v-verif#1729.

Once the end-of-test condition is detected, the end_of_test_o output of RVFI tracer is set and held indefinitely.

FORNOW: Upon hitting the programmable timeout, the RVFI tracer raises an end-of-test condition with the test exit code value 31'h7fff_ffff, equivalent to writing 32'hffff_ffff into tohost. Once the timeout feature is moved outside of the RVFI tracer, this behavior may be removed.