Add variable to track bats call arguments

[soda480]

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Having such a variable is useful for use cases where downstream functions can call bats with the original arguments, for example I am defining a setup_suite to automatically generate a coverage report and badge, I can use this variable like:

coverage() {
    if which kcov; then
        echo "# generating coverage report using kcov" >&3
        # strip --setup-suite-file argument to prevent infinite loop condition
        kcov --include-path=$WORK_DIR $WORK_DIR/coverage bats ${BATS_ARGS%--setup-suite-file*}
        coverage_badge
    else
        echo "# unable to generate coverage report because kcov is not installed" >&3
    fi
}