[!684] add explicit capture for `this`

Merge branch 'explicit-capture-for-this' into 'master'

ref:core/dune-common 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]

See merge request [!684]

  [!684]: gitlab.dune-project.org/core/dune-common/merge_requests/684