Drop civet/conduit from dev dependencies

These don't seem to be maintained anymore, and they brought a lot of
weight for just one test. I replaced them with a bare-bones http/cgi
server using just the standard library.

Author: ehuss

Cargo.lock

@@ -19,9 +19,6 @@ log = "0.4"
 git2 = { path = "..", version = "0.20", default-features = false }
 
 [dev-dependencies]
-civet = "0.11"
-conduit = "0.8"
-conduit-git-http-backend = "0.8"
 tempfile = "3.0"
 
 [features]