CMake: Add a subdirectory build regression test

If we have a regression with the "build Mbed TLS as a subdirectory with
CMake" feature and fail to build, fail the test.
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index 9d628f8..d240ddb 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -242,6 +242,11 @@
     git checkout -- Makefile library/Makefile programs/Makefile tests/Makefile
     cd ..
 
+    # Remove any artifacts from the component_test_cmake_as_subdirectory test.
+    rm -rf programs/test/cmake_subproject/build
+    rm -f programs/test/cmake_subproject/Makefile
+    rm -f programs/test/cmake_subproject/cmake_subproject
+
     if [ -f "$CONFIG_BAK" ]; then
         mv "$CONFIG_BAK" "$CONFIG_H"
     fi
@@ -1219,6 +1224,19 @@
     unset MBEDTLS_ROOT_DIR
 }
 
+component_test_cmake_as_subdirectory () {
+    msg "build: cmake 'as-subdirectory' build"
+    MBEDTLS_ROOT_DIR="$PWD"
+
+    cd programs/test/cmake_subproject
+    cmake .
+    make
+    if_build_succeeded ./cmake_subproject
+
+    cd "$MBEDTLS_ROOT_DIR"
+    unset MBEDTLS_ROOT_DIR
+}
+
 component_test_zeroize () {
     # Test that the function mbedtls_platform_zeroize() is not optimized away by
     # different combinations of compilers and optimization flags by using an