test: fix problem with threads test using default library context.

Also add a new test that deliberately tests the default library context.

Fixes #14720

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14773)

test/threadstest.c

@@ -348,7 +348,7 @@ static void thread_general_worker(void)
 
 static void thread_multi_simple_fetch(void)
 {
-    EVP_MD *md = EVP_MD_fetch(NULL, "SHA2-256", NULL);
+    EVP_MD *md = EVP_MD_fetch(multi_libctx, "SHA2-256", NULL);
 
     if (md != NULL)
         EVP_MD_free(md);
@@ -501,6 +501,7 @@ static int test_multi(int idx)
     OSSL_LIB_CTX_free(multi_libctx);
     EVP_PKEY_free(shared_evp_pkey);
     shared_evp_pkey = NULL;
+    multi_libctx = NULL;
     return testresult;
 }
 
@@ -532,6 +533,36 @@ static int test_multi_load(void)
     return 1;
 }
 
+static int test_multi_default(void)
+{
+    thread_t thread1, thread2;
+    int testresult = 0;
+    OSSL_PROVIDER *prov = NULL;
+
+    multi_success = 1;
+    multi_libctx = NULL;
+    prov = OSSL_PROVIDER_load(multi_libctx, "default");
+    if (!TEST_ptr(prov))
+        goto err;
+
+    if (!TEST_true(run_thread(&thread1, thread_multi_simple_fetch))
+            || !TEST_true(run_thread(&thread2, thread_multi_simple_fetch)))
+        goto err;
+
+    thread_multi_simple_fetch();
+
+    if (!TEST_true(wait_for_thread(thread1))
+            || !TEST_true(wait_for_thread(thread2))
+            || !TEST_true(multi_success))
+        goto err;
+
+    testresult = 1;
+
+ err:
+    OSSL_PROVIDER_unload(prov);
+    return testresult;
+}
+
 typedef enum OPTION_choice {
     OPT_ERR = -1,
     OPT_EOF = 0,
@@ -573,6 +604,9 @@ int setup_tests(void)
     if (!TEST_ptr(privkey))
         return 0;
 
+    /* Keep first to validate auto creation of default library context */
+    ADD_TEST(test_multi_default);
+
     ADD_TEST(test_lock);
     ADD_TEST(test_once);
     ADD_TEST(test_thread_local);