Fix failure when CFLAGS contains -DNDEBUG (Bug 25251)

Fix failure when CFLAGS contains -DNDEBUG (Bug 25251)

Building tests with -DNDEBUG in CFLAGS, gcc 9.2.1 issues the following error:
tst-assert-c++.cc: In function ‘int do_test()’:
tst-assert-c++.cc:66:12: error: unused variable ‘value’ [-Werror=unused-variable]
   66 |     no_int value;
      |            ^~~~~
tst-assert-c++.cc:71:18: error: unused variable ‘value’ [-Werror=unused-variable]
   71 |     bool_and_int value;
      |                  ^~~~~

The assert has been disabled by building glibc with CFLAGS, CXXFLAGS,
and CPPFLAGS with -DNDEBUG which removes the assert and leaves the
value unused.

We never want the assert disabled because that's the point of the
test, so we undefine NDEBUG before including assert.h to ensure that
we get assert correctly defined.