Merge bitcoin-core/secp256k1#1186: tests: Tidy context tests

39e8f0e refactor: Separate run_context_tests into static vs proper contexts (Tim Ruffing)
a4a0937 tests: Clean up and improve run_context_tests() further (Tim Ruffing)
fc90bb5 refactor: Tidy up main() (Tim Ruffing)
f32a36f tests: Don't use global context for context tests (Tim Ruffing)
ce4f936 tests: Tidy run_context_tests() by extracting functions (Tim Ruffing)
18e0db3 tests: Don't recreate global context in scratch space test (Tim Ruffing)
b198061 tests: Use global copy of secp256k1_context_static instead of clone (Tim Ruffing)

Pull request description:

  This is an improved version of some of the tidying/refactoring in bitcoin#1170.

  I think it's enough to deserve a separate PR. Once this is merged, I'll get back to the actual goal of bitcoin#1170 (namely, forbidding cloning and randomizing static contexts.)

  This PR is a general clean up of the context tests. A notable change is that this avoids a code smell where `run_context_tests()` would use the global `ctx` variable like a local one (i.e., create a context in it and destroy it afterwards).  After this PR, the global `ctx` is properly initialized for all the other tests, and they can decide whether they want to use it or not. Same for a global `sttc`, which is a memcpy of the static context (we need a writable copy in order to be able to set callbacks).

  Note that this touches code which is also affected by bitcoin#1167 but I refrained from trying to solve this issue. The goal of this PR is simply not to worsen the situation w.r.t. bitcoin#1167. We should really introduce a macro to solve bitcoin#1167 but that's another PR.

ACKs for top commit:
  sipa:
    utACK 39e8f0e
  apoelstra:
    ACK 39e8f0e

Tree-SHA512: a22471758111061a062b126a52a0de24a1a311d1a0332a4ef006882379a4f3f2b00e53089e3c374bf47c4051bb10bbc6a9fdbcf6d0cd4eca15b5703590395fba

src/modules/extrakeys/tests_impl.h

@@ -336,7 +336,6 @@ void test_keypair(void) {
     secp256k1_xonly_pubkey xonly_pk, xonly_pk_tmp;
     int pk_parity, pk_parity_tmp;
     int ecount;
-    secp256k1_context *sttc = secp256k1_context_clone(secp256k1_context_static);
 
     set_counting_callbacks(ctx, &ecount);
     set_counting_callbacks(sttc, &ecount);
@@ -440,7 +439,9 @@ void test_keypair(void) {
     memset(&keypair, 0, sizeof(keypair));
     CHECK(secp256k1_keypair_sec(ctx, sk_tmp, &keypair) == 1);
     CHECK(secp256k1_memcmp_var(zeros96, sk_tmp, sizeof(sk_tmp)) == 0);
-    secp256k1_context_destroy(sttc);
+
+    secp256k1_context_set_error_callback(sttc, NULL, NULL);
+    secp256k1_context_set_illegal_callback(sttc, NULL, NULL);
 }
 
 void test_keypair_add(void) {

src/modules/recovery/tests_impl.h

@@ -30,7 +30,6 @@ static int recovery_test_nonce_function(unsigned char *nonce32, const unsigned c
 
 void test_ecdsa_recovery_api(void) {
     /* Setup contexts that just count errors */
-    secp256k1_context *sttc = secp256k1_context_clone(secp256k1_context_static);
     secp256k1_pubkey pubkey;
     secp256k1_pubkey recpubkey;
     secp256k1_ecdsa_signature normal_sig;
@@ -124,7 +123,8 @@ void test_ecdsa_recovery_api(void) {
     CHECK(ecount == 7);
 
     /* cleanup */
-    secp256k1_context_destroy(sttc);
+    secp256k1_context_set_error_callback(sttc, NULL, NULL);
+    secp256k1_context_set_illegal_callback(sttc, NULL, NULL);
 }
 
 void test_ecdsa_recovery_end_to_end(void) {

src/modules/schnorrsig/tests_impl.h

@@ -128,8 +128,7 @@ void test_schnorrsig_api(void) {
     secp256k1_schnorrsig_extraparams invalid_extraparams = {{ 0 }, NULL, NULL};
 
     /** setup **/
-    secp256k1_context *sttc = secp256k1_context_clone(secp256k1_context_static);
-    int ecount;
+    int ecount = 0;
 
     secp256k1_context_set_error_callback(ctx, counting_illegal_callback_fn, &ecount);
     secp256k1_context_set_illegal_callback(ctx, counting_illegal_callback_fn, &ecount);
@@ -198,7 +197,8 @@ void test_schnorrsig_api(void) {
     CHECK(secp256k1_schnorrsig_verify(ctx, sig, msg, sizeof(msg), &zero_pk) == 0);
     CHECK(ecount == 4);
 
-    secp256k1_context_destroy(sttc);
+    secp256k1_context_set_error_callback(sttc, NULL, NULL);
+    secp256k1_context_set_illegal_callback(sttc, NULL, NULL);
 }
 
 /* Checks that hash initialized by secp256k1_schnorrsig_sha256_tagged has the