test: fix memory leaks in inspector tests

The inspector tests weren't closing open libuv handles properly,
making valgrind complain.  Strengthen the uv_loop_close() check
while here.

With this commit applied:

    $ valgrind ./out/Release/cctest 2>&1 | grep 'total heap' | cut -c31-
    1,017 allocs, 1,017 frees, 21,695,456 bytes allocated

PR-URL: https://github.com/nodejs/node/pull/7906


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>