test_value.py: _inject_numeric_testing_methods(): remove `has_neg`

test_value.py: _inject_numeric_testing_methods(): remove `has_neg`

This was only required for "self", __i*__() operators, which needed to
modify the value object. For example, you couldn't do:

    u = bt2.UnsignedIntegerValue(34)
    u -= 100

because the assignment in __isub__() had an invalid value.

Now the __i*__() operators do not exist anymore, so all RHS negative
values work.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ib2217150bd5c90b2ab72e5ed3b02c5d5206a22c3
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1585
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>