Errno fix (#463)

Correctly set errno on failure and improve the related test.

Previously the malloc test would emit an error message but not
abort if the errno was not as expected on failure. This
was because the return in the null == true case prevented the
check for failed == true at the end of check_result from
being reached. To resolve this just abort immediately as in the 
null case.

Also add tests of allocations that are expected to fail for
calloc and malloc.

To make the tests pass we need to set errno in several places,
making sure to keep this off the fast path.

We must also take care not to attempt to zero nullptr in case
of calloc failure.

See microsoft/snmalloc#461 and microsoft/snmalloc#463.

src/mem/external_alloc.h

@@ -56,15 +56,20 @@ namespace snmalloc::external_alloc
     if constexpr (bits::BITS == 64)
     {
       if (size >= 0x10000000000)
+      {
+        errno = ENOMEM;
         return nullptr;
+      }
     }
 
     if (alignment < sizeof(void*))
       alignment = sizeof(void*);
 
     void* result;
-    if (posix_memalign(&result, alignment, size) != 0)
+    int err = posix_memalign(&result, alignment, size);
+    if (err != 0)
     {
+      errno = err;
       result = nullptr;
     }
     return result;