tests: don't use $VERBOSE

The CI runs "make check VERBOSE=1".

commit 1ae4f1d3291b22466a20d1a2cce88b750ffbfb31
Author: Akim Demaille <[email protected]>
Date:   Mon Jun 29 06:44:42 2020 +0200

    tests: don't use $VERBOSE
    
    It is used by the test suite itself, which results in this test
    failing.
    
    * tests/c++.at: Use $DEBUG, not $VERBOSE.

diff --git a/tests/c++.at b/tests/c++.at
index 5cbbc91e..949bdeb6 100644
--- a/tests/c++.at
+++ b/tests/c++.at
@@ -46,7 +46,7 @@ template <typename T>
 bool
 check (const T& in, const std::string& s)
 {
-  const static bool verbose = getenv ("VERBOSE");
+  const static bool verbose = getenv ("DEBUG");
   std::stringstream os;
   os << in;
   if (os.str () == s)