tests: Added GTests for base/bitfield.hh

tests: Added GTests for base/bitfield.hh

In addition to the tests, a more detailed explanation of how
"insertBits(..)" functions has been included in its doxygen
documentation. The previous explanation was ambigious and led to
confusion.

Change-Id: I2ae8608733ebaa8f8f726cbb3a2cd8639b69c6b7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21700
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Jason Lowe-Power <jason@lowepower.com>