unity: flush test name before starting the test, use printf everywhere

unity: flush test name before starting the test, use printf everywhere

Unit test runner expects to see the test name echoed after test
selection is made. If the unit test immediately goes into sleep mode,
UART output will not be complete, and test runner will not see the
test name. This flushes the stream buffer and waits for UART FIFO to
be empty before starting the test.

Additionally some parts of code used unity_printf and some used
printf. Since unity_printf was only useful during very early tests
when newlib was not available, replace all its usages with printf.