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/library/common.c b/library/common.c
new file mode 100644
index 0000000..9f901f0
--- /dev/null
+++ b/library/common.c
@@ -0,0 +1,27 @@
+/*
+ *  Internal invasive testing helper functions
+ *
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may
+ *  not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+
+#if defined(MBEDTLS_TEST_HOOKS)
+void (*mbedtls_test_err_add_hook)( int, int, const char *, int );
+int mbedtls_err_add( int high, int low, const char *file, int line ) {
+    if( mbedtls_test_err_add_hook != NULL )
+        (*mbedtls_test_err_add_hook)( high, low, file, line );
+    return ( high + low );
+}
+#endif