tests: Support multiple HTTP server instances.

* guix/tests/http.scm (%http-server-socket): Turn into...
(open-http-server-socket): ... this procedure.
(http-server-can-listen?): New procedure.
(http-write, %http-server-lock, %http-server-ready)
(http-open, stub-http-server): Move to 'call-with-http-server' body.
(call-with-http-server): Add #:headers parameter.
(with-http-server): Add an additional pattern with headers.
* tests/derivations.scm: Use (http-server-can-listen?) instead
of (force %http-server-socket).
* tests/lint.scm: Likewise.

tests/derivations.scm

@@ -222,7 +222,7 @@
       (build-derivations %store (list drv))
       #f)))
 
-(unless (force %http-server-socket)
+(unless (http-server-can-listen?)
   (test-skip 1))
 (test-assert "'download' built-in builder"
   (let ((text (random-text)))
@@ -238,7 +238,7 @@
                          get-string-all)
                        text))))))
 
-(unless (force %http-server-socket)
+(unless (http-server-can-listen?)
   (test-skip 1))
 (test-assert "'download' built-in builder, invalid hash"
   (with-http-server 200 "hello, world!"
@@ -253,7 +253,7 @@
         (build-derivations %store (list drv))
         #f))))
 
-(unless (force %http-server-socket)
+(unless (http-server-can-listen?)
   (test-skip 1))
 (test-assert "'download' built-in builder, not found"
   (with-http-server 404 "not found"
@@ -279,7 +279,7 @@
       (build-derivations %store (list drv))
       #f)))
 
-(unless (force %http-server-socket)
+(unless (http-server-can-listen?)
   (test-skip 1))
 (test-assert "'download' built-in builder, check mode"
   ;; Make sure rebuilding the 'builtin:download' derivation in check mode