A `cmd` feature for custom shell-based checks

In addition to harcoded features in varnishtest, this opens a window for
out-of-tree uses of the test framework to skip test cases if an external
component (a database system, OS-specific capabilities, an environment
variable, a library, etc) is missing.

This feature takes an extra argument, a command-line that must exit with
a zero status. Complex feature testing can nicely be wrapped in scripts
at the user's discretion:

    feature cmd "my --command=line"

If the test is skipped, it is logged as:

    **   top   0.0 === feature cmd "my --command=line"
    *    top   0.0 SKIPPING test, missing feature: my --command=line

If the command-line is missing, it is logged as:

    **   top   0.0 === feature cmd
    ---- top   0.0 Missing the command line

bin/varnishtest/tests/a00014.vtc

+varnishtest "Custom feature verification"
+
+feature cmd true
+feature cmd false
+
+this is an invalid varnishtest command

bin/varnishtest/vtc.c

@@ -604,12 +604,14 @@ cmd_random(CMD_ARGS)
  *        The vcache user is present
  * group_varnish
  *        The varnish group is present
+ * cmd <command-line>
+ *        A command line that should execute with a zero exit status
  */
 
 static void
 cmd_feature(CMD_ARGS)
 {
-	int i;
+	int i, r;
 
 	(void)priv;
 	(void)cmd;
@@ -653,6 +655,15 @@ cmd_feature(CMD_ARGS)
 		    getgrnam("varnish") != NULL)
 			continue;
 
+		if (!strcmp(av[i], "cmd")) {
+			i++;
+			if (av[i] == NULL)
+				vtc_log(vl, 0, "Missing the command-line");
+			r = system(av[i]);
+			if (WEXITSTATUS(r) == 0)
+				continue;
+		}
+
 		vtc_log(vl, 1, "SKIPPING test, missing feature: %s", av[i]);
 		vtc_stop = 1;
 		return;