[zircon][core-tests] Nits.

Cpp Style (anonymous namespace for tests, and CamelCase for function
names).
Use zx type safe classes instead of raw zx_handle_t.
Prevent leak of handles by early release of a[0] on failure.

TEST=core/handle-transfer-test

Change-Id: Ic9ec5346d70afb9846f233c814ad709a3b35addd