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
diff --git a/library/common.h b/library/common.h
index 5845766..f4cef97 100644
--- a/library/common.h
+++ b/library/common.h
@@ -29,6 +29,7 @@
 #include "mbedtls/config.h"
 #endif
 
+#if defined(MBEDTLS_TEST_HOOKS)
 /** Helper to define a function as static except when building invasive tests.
  *
  * If a function is only used inside its own source file and should be
@@ -44,10 +45,25 @@
  * #endif
  * ```
  */
-#if defined(MBEDTLS_TEST_HOOKS)
 #define MBEDTLS_STATIC_TESTABLE
+
+/** Helper macro and function to combine a high and low level error code.
+ * 
+ * This function uses a hook (`mbedtls_test_err_add_hook`) to allow invasive
+ * testing of its inputs. This is used in the test infrastructure to report
+ * on errors when combining two error codes of the same level (e.g: two high
+ * or two low level errors).
+ */
+int mbedtls_err_add( int high, int low, const char *file, int line );
+#define MBEDTLS_ERR_ADD( high, low )  \
+    ( mbedtls_err_add( high, low, __FILE__, __LINE__ ) )
+
 #else
 #define MBEDTLS_STATIC_TESTABLE static
-#endif
+
+#define MBEDTLS_ERR_ADD( high, low ) \
+    ( high + low )
+
+#endif /* MBEDTLS_TEST_HOOKS */
 
 #endif /* MBEDTLS_LIBRARY_COMMON_H */
diff --git a/library/error.c b/library/error.c
index 486afed..901a369 100644
--- a/library/error.c
+++ b/library/error.c
@@ -210,13 +210,6 @@
 #include "mbedtls/xtea.h"
 #endif
 
-#if defined(MBEDTLS_TEST_HOOKS)
-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
 
 const char * mbedtls_high_level_strerr( int error_code )
 {