`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>
