tests: Convert remaining "sys.exit()" to "raise SystemExit".

tests/extmod/btree1.py

tests/extmod/framebuf1.py

tests/extmod/framebuf16.py

tests/extmod/framebuf4.py

tests/extmod/machine1.py

tests/extmod/machine_pinbase.py

tests/extmod/machine_pulse.py

tests/extmod/machine_signal.py

tests/extmod/time_ms_us.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_32bit_intbig.py

tests/extmod/uctypes_array_assign_le.py

tests/extmod/uctypes_array_assign_native_le.py

tests/extmod/uctypes_array_assign_native_le_intbig.py

tests/extmod/uctypes_bytearray.py