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]

Note that pre-C++20 does not allow to explicitly capture `this` when
the default capture is `=`.  So all captures need to be listed.

dune/common/test/testsuite.hh

@@ -75,7 +75,7 @@ namespace Dune {
       if (not condition)
         ++failedChecks_;
 
-      return CollectorStream([=](std::string reason) {
+      return CollectorStream([condition, name, this](std::string reason) {
           if (not condition)
             this->announceCheckResult(throwPolicy_, "CHECK  ", name, reason);
         });
@@ -96,7 +96,7 @@ namespace Dune {
       if (not condition)
         ++failedChecks_;
 
-      return CollectorStream([=](std::string reason) {
+      return CollectorStream([condition, name, this](std::string reason) {
           if (not condition)
             this->announceCheckResult(true, "REQUIRED CHECK", name, reason);
         });