tests: Skip tests that would hit the shebang length limitation.

* tests/gexp.scm (shebang): New variable.
  Skip "gexp->script" when SHEBANG is longer than 127 chars.
* guix/gexp.scm (gexp->script): Add comment on the issue.

guix/gexp.scm

@@ -351,6 +351,10 @@ its search path."
                       (gexp
                        (call-with-output-file (ungexp output)
                          (lambda (port)
+                           ;; Note: that makes a long shebang.  When the store
+                           ;; is /gnu/store, that fits within the 128-byte
+                           ;; limit imposed by Linux, but that may go beyond
+                           ;; when running tests.
                            (format port
                                    "#!~a/bin/guile --no-auto-compile~%!#~%"
                                    (ungexp guile))

tests/gexp.scm

@@ -211,6 +211,14 @@
     (return (string=? (readlink (string-append out "/foo"))
                       guile))))
 
+(define shebang
+  (string-append (derivation->output-path guile-for-build)
+                 "/bin/guile --no-auto-compile"))
+
+;; If we're going to hit the silly shebang limit (128 chars on Linux-based
+;; systems), then skip the following test.
+(test-skip (if (> (string-length shebang) 127) 1 0))
+
 (test-assertm "gexp->script"
   (mlet* %store-monad ((n ->   (random (expt 2 50)))
                        (exp -> (gexp