[microTVM] Fix test_autotune to support schedules with no tuning space

[guberti]

Currently, tests/micro/common/test_autotune.py passes on main when run on qemu or other microTVM devices (such as the Arduino Due) without the ARM DSP extension. However, when run on Cortex M4 or M7 devices with this extension, the following error is seen:

        logs = list(map(json.loads, str_logs))
>       assert len(logs) == 1 * TUNING_RUNS_PER_OPERATOR  # One operator
E       AssertionError: assert 3 == (1 * 2)
E        +  where 3 = len([{'config': {'code_hash': None, 'entity': [['tile_co', 'sp', [-1, 8]], ['tile_oh', 'sp', [-1, 5]], ['tile_ow', 'sp', [..., 'int32'], {}], 'result': [[0.001604125], 0, 14.074790239334106, 1660810737.063845], 'tvm_version': '0.10.dev0', ...}])

This is because there are two schedules for Arm with DSP, but one of them does not have any tuning space (giving 2 + 1 = 3 total autotuning uploads). This PR fixes the issue by checking the number of tasks extracted, and ensuring the number of tuned runs is in an expected range.

cc @alanmacd @gromero @mehrdadh