tests: Added GTests for byteswap.hh

tests: Added GTests for byteswap.hh

In addition to the test, "#include base/logging.hh" was added to the
"byteswap.hh". It is is required to compile the header.

Added tests ByteswapTest.swap_byte64, ByteswapTest.swap_byte32,
ByteswapTest.swap_byte16, ByteswapTest.swap_byte, ByteswapTest.htog,
and ByteswapTest.gtoh. The file byteswap.hh is mostly templates.

Added test for BigEndianGuest and LittleEndianGuest namespaces.

Change-Id: I8870a55594ed439fe9e1fb333384f73261d1b1b8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/22080
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>