ctest: Set baud rate to 115200

The baud rate used by all examples is 115200, which needs to be
specified when running tests with htrun. As there is more than one
common option to pass to htrun for all examples, add a new variable
IOTSDK_HTRUN_COMMON_ARGUMENTS to store the list of options to avoid
duplication.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>