hush: fix negative_arith.tests: glob-protect dash in "$((arith))"

function                                             old     new   delta
expand_vars_to_list                                 1026    1082     +56
parse_dollar                                         810     811      +1
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(add/remove: 0/0 grow/shrink: 2/0 up/down: 57/0)               Total: 57 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>