Restore MBEDTLS_TEST_OUTCOME_FILE after test_default_out_of_box

Since components run in the main process, unsetting
MBEDTLS_TEST_OUTCOME_FILE unset it in subsequent components as well.
To avoid this, save and restore the value.

(Making each component run in a subshell would be a better solution,
but it would be a much bigger change.)
diff --git a/tests/scripts/all.sh b/tests/scripts/all.sh
index f320866..1ee4eb1 100755
--- a/tests/scripts/all.sh
+++ b/tests/scripts/all.sh
@@ -667,6 +667,7 @@
     msg "build: make, default config (out-of-box)" # ~1min
     make
     # Disable fancy stuff
+    SAVE_MBEDTLS_TEST_OUTCOME_FILE="$MBEDTLS_TEST_OUTCOME_FILE"
     unset MBEDTLS_TEST_OUTCOME_FILE
 
     msg "test: main suites make, default config (out-of-box)" # ~10s
@@ -674,6 +675,9 @@
 
     msg "selftest: make, default config (out-of-box)" # ~10s
     programs/test/selftest
+
+    export MBEDTLS_TEST_OUTCOME_FILE="$SAVE_MBEDTLS_TEST_OUTCOME_FILE"
+    unset SAVE_MBEDTLS_TEST_OUTCOME_FILE
 }
 
 component_test_default_cmake_gcc_asan () {