Extend the test_multi_load() test

Run more threads and load the legacy provider (which uses a child lib ctx)
in order to hit more possible thread failures.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16980)

test/threadstest.c

diff --git a/test/threadstest.c b/test/threadstest.c
index f689676..cfd5991 100644
--- a/test/threadstest.c
+++ b/test/threadstest.c
@@ -30,7 +30,7 @@
 #include "threadstest.h"
 
 /* Limit the maximum number of threads */
-#define MAXIMUM_THREADS     3
+#define MAXIMUM_THREADS     10
 
 /* Limit the maximum number of providers loaded into a library context */
 #define MAXIMUM_PROVIDERS   4
@@ -558,6 +558,7 @@
     return testresult;
 }
 
+static char *multi_load_provider = "legacy";
 /*
  * This test attempts to load several providers at the same time, and if
  * run with a thread sanitizer, should crash if the core provider code
@@ -567,7 +568,7 @@
 {
     OSSL_PROVIDER *prov;
 
-    if (!TEST_ptr(prov = OSSL_PROVIDER_load(multi_libctx, "default"))
+    if (!TEST_ptr(prov = OSSL_PROVIDER_load(multi_libctx, multi_load_provider))
             || !TEST_true(OSSL_PROVIDER_unload(prov)))
         multi_success = 0;
 }
@@ -588,6 +589,7 @@
 static int test_multi_load(void)
 {
     int res = 1;
+    OSSL_PROVIDER *prov;
 
     /* The multidefault test must run prior to this test */
     if (!multidefault_run) {
@@ -595,7 +597,21 @@
         res = test_multi_default();
     }
 
-    return thread_run_test(NULL, 3, &test_multi_load_worker, 0, NULL) && res;
+    /*
+     * We use the legacy provider in test_multi_load_worker because it uses a
+     * child libctx that might hit more codepaths that might be sensitive to
+     * threading issues. But in a no-legacy build that won't be loadable so
+     * we use the default provider instead.
+     */
+    prov = OSSL_PROVIDER_load(NULL, "legacy");
+    if (prov == NULL) {
+        TEST_info("Cannot load legacy provider - assuming this is a no-legacy build");
+        multi_load_provider = "default";
+    }
+    OSSL_PROVIDER_unload(prov);
+
+    return thread_run_test(NULL, MAXIMUM_THREADS, &test_multi_load_worker, 0,
+                          NULL) && res;
 }
 
 static void test_obj_create_one(void)