cocotb_test/simulator.py: fix exit gracefully

[akukulanski]

Hi,
This small fix avoids the exception during the call to exit_gracefully() and prevents the simulator to keep running in the background after sensing a SIGINT.

================================================================================================================= FAILURES =================================================================================================================
_____________________________________________________________________________________________________________ test_axi_device ______________________________________________________________________________________________________________

self = <nmigen_cocotb.Icarus_g2005 object at 0x7ff14b667100>
cmds = [['iverilog', '-o', '/tmp/tmphv4tc1qb/top.vvp', '-D', 'COCOTB_SIM=1', '-s', ...], ['vvp', '-M', 'xxx...sim-model/test_venv/lib/python3.9/site-packages/cocotb/libs', '-m', 'libcocotbvpi_icarus', '/tmp/tmphv4tc1qb/top.vvp']]

    def execute(self, cmds):
        self.set_env()
        for cmd in cmds:
            self.logger.info("Running command: " + " ".join(cmd))
    
            with subprocess.Popen(
                cmd,
                stdout=subprocess.PIPE,
                stderr=subprocess.STDOUT,
                cwd=self.work_dir,
                env=self.env
            ) as p:
>               for line in p.stdout:

test_venv/lib/python3.9/site-packages/cocotb_test/simulator.py:263: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <nmigen_cocotb.Icarus_g2005 object at 0x7ff14b667100>, signum = 2
frame = <frame at 0x7ff14b7879f0, file 'xxx...sim-model/test_venv/lib/python3.9/site-packages/cocotb_test/simulator.py', line 264, code execute>

    def exit_gracefully(self, signum, frame):
        pid = None
>       if self.process is not None:
E       AttributeError: 'Icarus_g2005' object has no attribute 'process'

test_venv/lib/python3.9/site-packages/cocotb_test/simulator.py:296: AttributeError

[themperek]

Thank you!