ci: Check that #include "*.h" works as a single include

Lesser form of include-what-you-use: at least guarantees that header
file did not forget to include something through some other included
file.

Activate run_single_includes_tests on CI.
Fix some IWYU violations.

References #5321

src/nvim/ex_docmd.c

@@ -67,6 +67,7 @@
 #include "nvim/event/rstream.h"
 #include "nvim/event/wstream.h"
 #include "nvim/shada.h"
+#include "nvim/globals.h"
 
 static int quitmore = 0;
 static int ex_pressedreturn = FALSE;