Replace .S test for bs_test_unit with .erl test

[pguyot]

These changes are made under both the "Apache 2.0" and the "GNU Lesser General
Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later

[bettio]

Fails on OTP-29 rc1:

test_op_bs_test_unit:
CRASH 
======
pid: <0.1.0>

Stacktrace:
[{test_op_bs_test_unit,test_unit_16,0,[{file,"/home/runner/work/AtomVM/AtomVM/tests/erlang_tests/test_op_bs_test_unit.erl"},{line,53}]},{test_op_bs_test_unit,start,0,[{file,"/home/runner/work/AtomVM/AtomVM/tests/erlang_tests/test_op_bs_test_unit.erl"},{line,36}]}]

cp: #CP<module: 0, label: 9, offset: 529>

x[0]: error
x[1]: {badmatch,<<2>>}
x[2]: {2,2,75,1,[{0,310},{0,2111}],error}

Stack 
-----

#CP<module: 0, label: 2, offset: 120>
#CP<module: 0, label: 0, offset: 125>


Mailbox
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Monitors
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**End Of Crash Report**
Expected 0 but result is not an integer
test_op_bs_test_unit:FAILED

[pguyot]

I suspect OTP29 to not be able to emit this opcode as the no_bs_match option is gone

OTP29 emits this opcode but eventually this test uncovered a bug in bs_match ensure_at_least which this PR now fixes in the first commit.