tests/utils/utils.sh: use "private" variables if applicable

tests/utils/utils.sh: use "private" variables if applicable

The non-exported `BT_PLUGINS_PATH` and `BT_TESTS_PYTHON_VERSION`
variables aren't directly needed by whatever sources `utils.sh` and its
subprocesses.

`BT_PLUGINS_PATH` is actually exported when calling run_python_bt2(),
for that specific case.

Make them more "private" by using the `_bt_tests_` prefix.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ic48bb1de3f4d45fa39692241872bd2e7969ec858