tests/extmod: Make tests skippable.

tests/extmod/machine1.py

tests/extmod/ubinascii_a2b_base64.py

tests/extmod/ubinascii_b2a_base64.py

tests/extmod/ubinascii_crc32.py

tests/extmod/ubinascii_hexlify.py

tests/extmod/ubinascii_micropython.py

tests/extmod/ubinascii_unhexlify.py

tests/extmod/uctypes_array_assign_le.py

tests/extmod/uctypes_array_assign_native_le.py

tests/extmod/uctypes_bytearray.py

tests/extmod/uctypes_le.py

tests/extmod/uctypes_le_float.py

tests/extmod/uctypes_native_float.py

tests/extmod/uctypes_native_le.py

tests/extmod/uctypes_print.py

tests/extmod/uctypes_ptr_le.py

tests/extmod/uctypes_ptr_native_le.py

tests/extmod/uctypes_sizeof.py

tests/extmod/uctypes_sizeof_native.py

tests/extmod/uheapq1.py