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>

src/base/SConscript

@@ -37,6 +37,7 @@ SimObject('Graphics.py')
 Source('atomicio.cc')
 GTest('atomicio.test', 'atomicio.test.cc', 'atomicio.cc')
 Source('bitfield.cc')
+GTest('bitfield.test', 'bitfield.test.cc', 'bitfield.cc')
 Source('imgwriter.cc')
 Source('bmpwriter.cc')
 Source('callback.cc')