add test for get rss feed endpoint

[orthur]

first test for getFeedCtrl from server.go

@umputun I would add more tests, after going through your review comments

[orthur]

server_test.go runs inside feed-master/app/api directory, hence can't find templates.

ok  	github.com/umputun/feed-master/app	0.008s	coverage: 5.1% of statements
panic: html/template: pattern matches no files: `webapp/templates/*`

goroutine 1 [running]:
html/template.Must(...)
	/opt/hostedtoolcache/go/1.17.7/x64/src/html/template/template.go:374
github.com/umputun/feed-master/app/api.init()
	/home/runner/work/feed-master/feed-master/app/api/web.go:18 +0x65
FAIL	github.com/umputun/feed-master/app/api	0.010s

should templates set by absolute path?
also found this workaround

[umputun]

I have changed the server a little bit to make it more testable. Added a simple test as well

[orthur]

I have added omitempty for image tags, without it feed would have empty URLs