iconv: Use TIMEOUTFACTOR for iconv test timeout

Currently the timeout for each iconv test is hard coded to 3 seconds.
On my OpenRISC test platform this is too slow and the test fails with a
HANG error.

This change uses the available TIMEOUTFACTOR to compute the timeout.
The default value is still 3.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>