patch 9.0.1627: no generic mechanism to test syntax plugins

Problem:    No generic mechanism to test syntax plugins.
Solution:   Add a syntax plugin test mechanism, using screendumps.  Add a
            simple test for "c".

runtime/syntax/testdir/input/c.c

@@ -0,0 +1,122 @@
+/* vi:set ts=8 sts=4 sw=4 noet:
+ *
+ * VIM - Vi IMproved	by Bram Moolenaar
+ *
+ * Do ":help uganda"  in Vim to read copying and usage conditions.
+ * Do ":help credits" in Vim to see a list of people who contributed.
+ * See README.txt for an overview of the Vim source code.
+ */
+
+#define EXTERN
+#include "vim.h"
+
+#ifdef __CYGWIN__
+# include <cygwin/version.h>
+# include <sys/cygwin.h>	// for cygwin_conv_to_posix_path() and/or
+				// cygwin_conv_path()
+# include <limits.h>
+#endif
+
+#if defined(MSWIN) && (!defined(FEAT_GUI_MSWIN) || defined(VIMDLL))
+# include "iscygpty.h"
+#endif
+
+// Values for edit_type.
+#define EDIT_NONE   0	    // no edit type yet
+#define EDIT_FILE   1	    // file name argument[s] given, use argument list
+#define EDIT_STDIN  2	    // read file from stdin
+#define EDIT_TAG    3	    // tag name argument given, use tagname
+#define EDIT_QF	    4	    // start in quickfix mode
+
+#if (defined(UNIX) || defined(VMS)) && !defined(NO_VIM_MAIN)
+static int file_owned(char *fname);
+#endif
+static void mainerr(int, char_u *);
+static void early_arg_scan(mparm_T *parmp);
+#ifndef NO_VIM_MAIN
+static void usage(void);
+static void parse_command_name(mparm_T *parmp);
+static void command_line_scan(mparm_T *parmp);
+static void check_tty(mparm_T *parmp);
+static void read_stdin(void);
+static void create_windows(mparm_T *parmp);
+static void edit_buffers(mparm_T *parmp, char_u *cwd);
+static void exe_pre_commands(mparm_T *parmp);
+static void exe_commands(mparm_T *parmp);
+static void source_startup_scripts(mparm_T *parmp);
+static void main_start_gui(void);
+static void check_swap_exists_action(void);
+# ifdef FEAT_EVAL
+static void set_progpath(char_u *argv0);
+# endif
+#endif
+
+
+/*
+ * Different types of error messages.
+ */
+static char *(main_errors[]) =
+{
+    N_("Unknown option argument"),
+#define ME_UNKNOWN_OPTION	0
+    N_("Too many edit arguments"),
+#define ME_TOO_MANY_ARGS	1
+    N_("Argument missing after"),
+#define ME_ARG_MISSING		2
+    N_("Garbage after option argument"),
+#define ME_GARBAGE		3
+    N_("Too many \"+command\", \"-c command\" or \"--cmd command\" arguments"),
+#define ME_EXTRA_CMD		4
+    N_("Invalid argument for"),
+#define ME_INVALID_ARG		5
+};
+
+#ifndef PROTO		// don't want a prototype for main()
+
+// Various parameters passed between main() and other functions.
+static mparm_T	params;
+
+#ifdef _IOLBF
+static void *s_vbuf = NULL;		// buffer for setvbuf()
+#endif
+
+#ifndef NO_VIM_MAIN	// skip this for unittests
+
+static char_u *start_dir = NULL;	// current working dir on startup
+
+static int has_dash_c_arg = FALSE;
+
+# ifdef VIMDLL
+__declspec(dllexport)
+# endif
+    int
+# ifdef MSWIN
+VimMain
+# else
+main
+# endif
+(int argc, char **argv)
+{
+#if defined(STARTUPTIME) || defined(CLEAN_RUNTIMEPATH)
+    int		i;
+#endif
+
+    /*
+     * Do any system-specific initialisations.  These can NOT use IObuff or
+     * NameBuff.  Thus emsg2() cannot be called!
+     */
+    mch_early_init();
+
+    // Source startup scripts.
+    source_startup_scripts(&params);
+
+#if 0
+    /*
+     * Newer version of MzScheme (Racket) require earlier (trampolined)
+     * initialisation via scheme_main_setup.
+     */
+    return mzscheme_main();
+#else
+    return vim_main2();
+#endif
+}