Move `MBEDTLS_ERR_ADD` macro and function to `common.*`

`error.c` is a file generated from `error.h` and thus cannot contain the code
that was previously added. This commit fixes that issue by moving the
`MBEDTLS_ERR_ADD` macro and associated function and function pointer into
`common.h` and `common.c`.

Also fix a typo in `tests/include/test/helpers.h` where tabs were accidentally
used instead of spaces.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/tests/include/test/helpers.h b/tests/include/test/helpers.h
index 1fe25d8..a26f1ee 100644
--- a/tests/include/test/helpers.h
+++ b/tests/include/test/helpers.h
@@ -280,9 +280,9 @@
 
 #if defined(MBEDTLS_TEST_HOOKS)
 /**
- * \brief	Check that a pure high-level error code is being combined with a
- *			pure low-level error code as otherwise the resultant error code
- *			would be corrupted.
+ * \brief   Check that a pure high-level error code is being combined with a
+ *          pure low-level error code as otherwise the resultant error code
+ *          would be corrupted.
  */
 void mbedtls_test_err_add_check( int high, int low,
                                  const char *file, int line);