Tests: Added GTests for base/callback.cc

Change-Id: Icebd0fdec4be86e0f0fd86ef58f52ddbfdf8d714
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21619
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/SConscript

@@ -39,6 +39,7 @@ Source('bitfield.cc')
 Source('imgwriter.cc')
 Source('bmpwriter.cc')
 Source('callback.cc')
+GTest('callback.test', 'callback.test.cc', 'callback.cc')
 Source('cprintf.cc', add_tags='gtest lib')
 GTest('cprintf.test', 'cprintf.test.cc')
 Source('debug.cc')