add explicit capture for `this`
This addresses the following warnings from GCC 9:
dune/common/test/testsuite.hh:78:30: warning: implicit capture of ‘this’ via ‘[=]’ is deprecated in C++20 [-Wdeprecated]
dune/common/test/testsuite.hh:99:30: warning: implicit capture of ‘this’ via ‘[=]’ is deprecated in C++20 [-Wdeprecated]