Add documentation and minor style changes

Add doxygen style documentation to `mbedtls_test_fail`, `mbedtls_test_skip`,
`mbedtls_test_set_step` and `mbedtls_test_info_reset`. This should make it
easier to understand how the test infrastructure is used.

Also make some minor style changes to meet the coding standards and make it
more obvious that `mbedtls_test_info.step` was being incremented.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/tests/suites/target_test.function b/tests/suites/target_test.function
index 8afe70e..637a79d 100644
--- a/tests/suites/target_test.function
+++ b/tests/suites/target_test.function
@@ -384,7 +384,7 @@
     while ( 1 )
     {
         ret = 0;
-        mbedtls_test_info_reset();
+        mbedtls_test_info_reset( );
         data_len = 0;
 
         data = receive_data( &data_len );