vfmt: fix string interpolation formatting with multiple ',",\' and \" .

vlib/vweb/request_test.v

@@ -100,7 +100,7 @@ fn test_parse_multipart_form() {
 	file := 'bar.v'
 	ct := 'application/octet-stream'
 	contents := ['baz', 'buzz']
-	data := '--------------------------$boundary
+	data := "--------------------------$boundary
 Content-Disposition: form-data; name=\"${names[0]}\"; filename=\"$file\"
 Content-Type: $ct
 
@@ -110,7 +110,7 @@ Content-Disposition: form-data; name=\"${names[1]}\"
 
 ${contents[1]}
 --------------------------$boundary--
-'
+"
 	form, files := parse_multipart_form(data, boundary)
 	assert files == map{
 		names[0]: [FileData{

vlib/vweb/tests/vweb_test.v

@@ -213,12 +213,12 @@ fn test_http_client_multipart_form_data() ? {
 	name := 'foo'
 	ct := 'multipart/form-data; boundary=------------------------$boundary'
 	contents := 'baz buzz'
-	data := '--------------------------$boundary
+	data := "--------------------------$boundary
 Content-Disposition: form-data; name=\"$name\"
 
 $contents
 --------------------------$boundary--
-'
+"
 	mut x := http.fetch('http://127.0.0.1:$sport/form_echo',
 		method: .post
 		header: http.new_header(