test: suppress warning in test_environment.cc

Currently there is a compiler warning generated if a build defines
NDEBUG:

$ env CXXFLAGS='-DNDEBUG' make -j8 cctest
../test/cctest/test_environment.cc:
In function ‘void at_exit_js(void*)’:
../test/cctest/test_environment.cc:333:25: warning:
variable ‘obj’ set but not used [-Wunused-but-set-variable]
  333 |   v8::Local<v8::Object> obj = v8::Object::New(isolate);
      |                         ^~~

NDEBUG is currently not defined using the main branch but this
discovered when working on replacing OpenSSL 1.1.1 with OpenSSL 3.0.

This commit uses EXPECT statements instead of asserts to avoid the
warning.

PR-URL: https://github.com/nodejs/node/pull/38868


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>