Tweak test for go 1.16

Apparently later versions of go strip the leading path from file upload test

submit_test.go

@@ -434,7 +434,7 @@ Content-Disposition: form-data; name="version"
 
 1
 ------WebKitFormBoundarySsdgl8Nq9voFyhdO
-Content-Disposition: form-data; name="file"; filename="etc/passwd.txt"
+Content-Disposition: form-data; name="file"; filename="passwd.txt"
 
 file
 ------WebKitFormBoundarySsdgl8Nq9voFyhdO--