tests: Stop "using namespace std" in unittest/.

tests: Stop "using namespace std" in unittest/.

These are the historical "unit test"s, which aren't really unit tests,
they're actually complete builds of gem5 with main functions which run a
fairly specific test instead of a simulation. They test a single unit,
but they do it with all the other units in place and potentially
participating in the test.

Change-Id: Ib0ea68f26091a79992396d932627e4ce180f7825
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39565
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>