add linux gcc test ci

tests/main.cpp

@@ -0,0 +1,9 @@
+#define DOCTEST_CONFIG_IMPLEMENT
+
+#include "doctest/doctest.h"
+
+// doctest comments
+// 'function' : must be 'attribute' - see issue #182
+DOCTEST_MSVC_SUPPRESS_WARNING_WITH_PUSH(4007)
+int main(int argc, char** argv) { return doctest::Context(argc, argv).run(); }
+DOCTEST_MSVC_SUPPRESS_WARNING_POP