test-network.sh uses a deprecated arithmetic syntax

test-network.sh uses a deprecated bash-specific syntax to do arithmetic. This fails on systems like Ubuntu and Debian that use dash instead of bash for /bin/sh. test-network.sh should probably use the POSIX $((arithmetic_expr)) syntax instead.

Typical error from the log files:

/home/tlyu/src/tor/../chutney/tools/test-network.sh: 302: [: Illegal number: $[n_rounds+1]
Completed $[n_rounds+1] of 1 verify rounds.