tests: Add dlopen override to dwfl-proc-attach for old glibc/valgrind.

Some combination of old glibc and valgrind create unsuppressable memory
leak warnings when the process is run under valgrind, is multi-threaded
and uses dlopen. libdw will try to dlopen libdebuginfod be default.
So simply override dlopen and always return NULL to make sure
libdebuginfod is never loaded.  The dwfl-proc-attach test doesn't rely
on libdebuginfod anyway.

This was seen on the armbian buildbot which uses valgrind 3.14.0 and
glibc 2.28.

Signed-off-by: Mark Wielaard <mark@klomp.org>

tests/ChangeLog

@@ -1,3 +1,7 @@
+2020-10-31  Mark Wielaard  <mark@klomp.org>
+
+	* dwfl-proc-attach.c (dlopen): New external function override.
+
 2020-10-31  Mark Wielaard  <mark@klomp.org>
 
 	* test-wrapper.sh: Use =, not == for string compare.

tests/dwfl-proc-attach.c

@@ -107,4 +107,16 @@ main (int argc __attribute__ ((unused)),
   return (threads == 3) ? 0 : -1;
 }
 
+/* HACK. This is a simple workaround for a combination of old glibc
+   and valgrind. libdw will try to dlopen libdebuginfod this causes
+   some unsuppressable memory leak warnings when the process is
+   multi-threaded under valgrind because of some bad backtraces.
+   So simply override dlopen and always return NULL so libdebuginfod
+   (and libcurl) are never loaded.  This test doesn't rely on
+   libdebuginfod anyway.  */
+void *dlopen (void)
+{
+  return NULL;
+}
+
 #endif /* __linux__ */