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/test_suite_asn1parse.function b/tests/suites/test_suite_asn1parse.function
index 21b3ab6..47a4340 100644
--- a/tests/suites/test_suite_asn1parse.function
+++ b/tests/suites/test_suite_asn1parse.function
@@ -594,6 +594,7 @@
     unsigned char *p = input->x;
     const char *rest = description;
     unsigned long n;
+    unsigned int step = 0;
 
     TEST_EQUAL( mbedtls_asn1_get_sequence_of( &p, input->x + input->len,
                                               &head, tag ),
@@ -614,7 +615,7 @@
             cur = &head;
             while( *rest )
             {
-                mbedtls_test_set_step( mbedtls_test_info.step + 1 );
+                mbedtls_test_set_step( step );
                 TEST_ASSERT( cur != NULL );
                 TEST_EQUAL( cur->buf.tag, tag );
                 n = strtoul( rest, (char **) &rest, 0 );
@@ -625,6 +626,7 @@
                 if( *rest )
                     ++rest;
                 cur = cur->next;
+                ++step;
             }
             TEST_ASSERT( cur == NULL );
         }