Do "strict" heapcheck instead of "draconian" (#5635)

The draconian heapcheck turns up memory leaks outside our control, such
as ones in libc and in the heapchecker itself. I think for an automated
test it makes more sense for us to use the "strict" heapcheck, since
that already passes and should only report leaks that are within our
control.

tests.sh

@@ -44,7 +44,7 @@ build_cpp_tcmalloc() {
       PTHREAD_CFLAGS='-pthread -DGOOGLE_PROTOBUF_HEAP_CHECK_DRACONIAN' \
       check
   cd src
-  PPROF_PATH=/usr/bin/google-pprof HEAPCHECK=draconian ./protobuf-test
+  PPROF_PATH=/usr/bin/google-pprof HEAPCHECK=strict ./protobuf-test
 }
 
 build_cpp_distcheck() {