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.h

@@ -2,6 +2,7 @@
 #define NVIM_EX_DOCMD_H
 
 #include "nvim/ex_cmds_defs.h"
+#include "nvim/globals.h"
 
 // flags for do_cmdline()
 #define DOCMD_VERBOSE   0x01      // included command in error message