Internals: Fix misc internal coverage holes. No functional change intended.

include/verilated.cpp

@@ -2254,11 +2254,24 @@ void Verilated::flushCb(VerilatedVoidCb cb) VL_MT_SAFE {
     }
 }
 
+// When running internal code coverage (gcc --coverage, as opposed to
+// verilator --coverage), dump coverage data to properly cover failing
+// tests.
+#ifdef VL_GCOV
+extern "C" {
+void __gcov_flush();  // gcc sources gcc/gcov-io.h has the prototype
+}
+void vl_gcov_flush() { __gcov_flush(); }
+#else
+void vl_gcov_flush() {}
+#endif
+
 void Verilated::flushCall() VL_MT_SAFE {
     const VerilatedLockGuard lock(m_mutex);
     if (s_flushCb) (*s_flushCb)();
     fflush(stderr);
     fflush(stdout);
+    vl_gcov_flush();
 }
 
 const char* Verilated::productName() VL_PURE { return VERILATOR_PRODUCT; }