Add a test for external alternate timing implementation

Copy the original implementation
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index d0bd1eb..127f424 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -2716,6 +2716,15 @@
     make test
 }
 
+component_test_alt_timing() {
+    msg "build: alternate timing implementation"
+    scripts/config.py set MBEDTLS_TIMING_ALT
+    make lib TEST_TIMING_ALT_IMPL=1 CFLAGS="-I../tests/src/external_timing"
+
+    msg "test: MBEDTLS_TIMING_ALT - test suites"
+    make test TEST_TIMING_ALT_IMPL=1 CFLAGS="-I../tests/src/external_timing"
+}
+
 component_test_platform_calloc_macro () {
     msg "build: MBEDTLS_PLATFORM_{CALLOC/FREE}_MACRO enabled (ASan build)"
     scripts/config.py set MBEDTLS_PLATFORM_MEMORY