fix(tftf): Prevent UART from exiting early

Ensured all calls to exit_uart in 'expect' wait for the string
"Exiting tests." to prevent the uart from being closed before all of the
test result information has been written.

Signed-off-by: Nathan Dunne <Nathan.Dunne@arm.com>
Change-Id: I4793be8fd5a2bbf3e2a3a72e14b0d702d52e05e4
3 files changed