handle Timer without Clock

[htfab]

Using Timer without setting up a Clock raises an IndexError:

from ttboard import cocotb
from ttboard.cocotb.triggers import Timer

@cocotb.test()
async def test_timer(dut):
    await Timer(10, units='ms')

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

Output:

MPY: soft reboot
db init: 128240
user conf loaded: 130240
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_timer ***
DUT: T*** Test 'test_timer' FAIL: list index out of range ***
DUT: 1/1 tests failed
DUT: *** Summary ***
DUT: 	FAIL	test_timer	list index out of range

The exception happens at ttboard/cocotb/triggers/timer.py line 22:

tt-micropython-firmware/src/ttboard/cocotb/triggers/timer.py

Line 22 in bb061d2

fastest_clock = all_clocks[0]

This PR is a quick and dirty fix for this specific case when all_clocks is empty. There are some other edge cases where either self.time or some of the other clocks aren't a multiple of the fastest clock, but we can deal with them later.