print stack trace on all exceptions

[htfab]

Non-assert exceptions in the testbench code don't produce a traceback, which makes debugging harder. Example:

from ttboard import cocotb

@cocotb.test()
async def test_exception(dut):
    a = []
    dut._log.info(a[0])

if __name__ == '__main__':
    from ttboard.cocotb.dut import DUTWrapper   
    cocotb.get_runner().test(DUTWrapper())

Ouput:

MPY: soft reboot
db init: 135392
user conf loaded: 139680
ttboard.demoboard: Demoboard starting up in mode ASIC_RP_CONTROL
ttboard.pins.pins: Setting mode to ASIC_RP_CONTROL
ttboard.boot.rom: Got ROM data shuttle=tt04
repo=TinyTapeout/tinytapeout-04

ttboard.project_mux: Disable (selecting project 0)
ttboard.project_mux: Loading shuttle file /shuttles/tt04.json
ttboard.project_mux: Enable design tt_um_factory_test
ttboard.demoboard: Resetting system clock to default 1.25e+08Hz
ttboard.demoboard: Clocking at 10Hz
ttboard.demoboard: First time loading: Toggling project reset
ttboard.demoboard: Changing reset to output mode
DUT: *** Running Test 1/1: test_exception ***
DUT: T*** Test 'test_exception' FAIL: list index out of range ***
DUT: 1/1 tests failed
DUT: *** Summary ***
DUT: 	FAIL	test_exception	list index out of range