Merge bitcoin-core/secp256k1#1195: ctime_tests: improve output when CHECKMEM_RUNNING is not defined

8f51229 ctime_tests: improve output when CHECKMEM_RUNNING is not defined (Jonas Nick)

Pull request description:

  When seeing the output
  ```
  Unless compiled under msan, this test can only usefully be run inside valgrind.
  ```
  I thought that I would have to go back to the `configure` output to manually check if it was compiled under memsan to determine whether this test can be usefully run outside valgrind. But when we go into this branch then it was definitely not compiled under msan, which means that we can make the output clearer.

ACKs for top commit:
  sipa:
    utACK 8f51229
  real-or-random:
    utACK bitcoin-core/secp256k1@8f51229

Tree-SHA512: a4953a158b1375d8fc3a2ee29e7014c5399becf5f75ffd3765c0141861e092fbc120003e00dfd25ec54b92a466e133377b96d5a9f4017c100aaf64fb9a045df1

Author: sipa

src/ctime_tests.c

@@ -38,7 +38,7 @@ int main(void) {
     int ret, i;
 
     if (!SECP256K1_CHECKMEM_RUNNING()) {
-        fprintf(stderr, "Unless compiled under msan, this test can only usefully be run inside valgrind.\n");
+        fprintf(stderr, "This test can only usefully be run inside valgrind because it was not compiled under msan.\n");
         fprintf(stderr, "Usage: libtool --mode=execute valgrind ./ctime_tests\n");
         return 1;
     }