gdb/testsuite: avoid assigning lambdas in Python files

gdb/testsuite: avoid assigning lambdas in Python files

Fix flake8 warnings like this one:

    gdb/testsuite/gdb.perf/template-breakpoints.py:35:13: E731 do not assign a lambda expression, use a def

I chose to inline the lambdas in the expressions, since they are simple
enough and only used once each.

Change-Id: I46fac428a95da38f5a6a87e101be4da9fa4acc31
Approved-By: Tom Tromey <tom@tromey.com>