add push tests

Signed-off-by: Mohammed Al Sahaf <msaa1990@gmail.com>

caddytest/spec/http/push/spec.hurl

@@ -0,0 +1,115 @@
+# push initiates HTTP/2 PUSH_PROMISE frames; libcurl does not surface pushed
+# streams as headers, so we only validate that push configurations parse and
+# do not break the response.
+
+# Configure Caddy with a single push target
+POST http://localhost:2019/load
+Content-Type: text/caddyfile
+```
+{
+	skip_install_trust
+	http_port     9080
+	https_port    9443
+	local_certs
+}
+localhost {
+	root * {{indexed_root}}
+	push /style.css
+	file_server
+}
+```
+HTTP 200
+
+# push does not break the response
+GET https://localhost:9443/index.html
+[Options]
+insecure: true
+HTTP 200
+
+
+# Configure Caddy with multiple push targets
+POST http://localhost:2019/load
+Content-Type: text/caddyfile
+```
+{
+	skip_install_trust
+	http_port     9080
+	https_port    9443
+	local_certs
+}
+localhost {
+	root * {{indexed_root}}
+	push /index.txt
+	push /index.html
+	respond "Main"
+}
+```
+HTTP 200
+
+# multiple push targets do not break the response
+GET https://localhost:9443
+[Options]
+insecure: true
+HTTP 200
+[Asserts]
+body == "Main"
+
+
+# Configure Caddy with conditional push gated by a matcher
+POST http://localhost:2019/load
+Content-Type: text/caddyfile
+```
+{
+	skip_install_trust
+	http_port     9080
+	https_port    9443
+	local_certs
+}
+localhost {
+	root * {{indexed_root}}
+	@html path *.html
+	push @html /index.txt
+	file_server
+}
+```
+HTTP 200
+
+# matching path still serves correctly
+GET https://localhost:9443/index.html
+[Options]
+insecure: true
+HTTP 200
+
+# non-matching path also serves correctly
+GET https://localhost:9443/index.txt
+[Options]
+insecure: true
+HTTP 200
+
+
+# Configure Caddy with push driven by upstream Link header
+POST http://localhost:2019/load
+Content-Type: text/caddyfile
+```
+{
+	skip_install_trust
+	http_port     9080
+	https_port    9443
+	local_certs
+}
+localhost {
+	push
+	header Link "</preload.css>; rel=preload; as=style"
+	respond "Linked"
+}
+```
+HTTP 200
+
+# bare push parses and forwards the Link header to the client
+GET https://localhost:9443/
+[Options]
+insecure: true
+HTTP 200
+[Asserts]
+header "Link" contains "</preload.css>"
+body == "Linked"