Allow shell commands in client/server specs

With this we can move body checks closer to HTTP checks.

bin/varnishtest/tests/a00015.vtc

 varnishtest "Write a body to a file"
 
 server s1 {
+	# First, HTTP checks
 	rxreq
 	expect req.http.Content-Type == "text/plain"
+
+	# Then, payload checks
 	write_body req.txt
+	shell {grep -q request req.txt}
+
 	txresp -hdr "Content-Type: text/plain" -body response
 } -start
 
@@ -11,10 +16,12 @@ varnish v1 -vcl+backend {} -start
 
 client c1 {
 	txreq -req POST -hdr "Content-Type: text/plain" -body request
+
+	# First, HTTP checks
 	rxresp
 	expect resp.http.Content-Type == "text/plain"
+
+	# Then, payload checks
 	write_body resp.txt
+	shell {grep -q response resp.txt}
 } -run
-
-shell {grep -q request req.txt}
-shell {grep -q response resp.txt}

bin/varnishtest/vtc.c

@@ -441,8 +441,12 @@ cmd_varnishtest(CMD_ARGS)
  *
  * The vtc will fail if the return code of the shell is not 0.
  */
+/* SECTION: client-server.spec.shell shell
+ *
+ * Same as for the top-level shell.
+ */
 
-static void
+void
 cmd_shell(CMD_ARGS)
 {
 	(void)priv;
@@ -471,8 +475,12 @@ cmd_shell(CMD_ARGS)
  * err_shell expect the shell command to fail AND stdout to match the string,
  * failing the test case otherwise.
  */
+/* SECTION: client-server.spec.shell err_shell
+ *
+ * Same as for the top-level err_shell.
+ */
 
-static void
+void
 cmd_err_shell(CMD_ARGS)
 {
 	(void)priv;

bin/varnishtest/vtc.h

@@ -66,6 +66,8 @@ CMD(varnish);
 CMD(barrier);
 CMD(logexpect);
 CMD(process);
+CMD(err_shell);
+CMD(shell);
 #undef CMD
 
 extern volatile sig_atomic_t vtc_error; /* Error, bail out */

bin/varnishtest/vtc_http.c

@@ -1860,6 +1860,8 @@ static const struct cmds http_cmds[] = {
 	/* general purpose */
 	CMD(barrier),
 	CMD(delay),
+	CMD(err_shell),
+	CMD(shell),
 #undef CMD_HTTP
 #undef CMD
 	{ NULL, NULL }