shell tests, windows: enable more tests

Enable on Windows:
//src/test/shell/bazel:bazel_experimental_ui_test
//src/test/shell/bazel:bazel_random_characters_test

Also document the (surprising) semantics of
http_file() on Windows.

See https://github.com/bazelbuild/bazel/issues/4292

Change-Id: Id636b9cf03cc8d12449eed2be493dcfc176146c7

Closes #5409.

Change-Id: I15677548f2f3fd6b4822e8e6af4b78d8517f953a
PiperOrigin-RevId: 201512812

tools/build_defs/repo/http.bzl

diff --git a/tools/build_defs/repo/http.bzl b/tools/build_defs/repo/http.bzl
index fffe226..727d3a7 100644
--- a/tools/build_defs/repo/http.bzl
+++ b/tools/build_defs/repo/http.bzl
@@ -316,6 +316,11 @@
 
   Targets would specify <code>@my_ssl//jar</code> as a dependency to depend on this jar.
 
+  You may also reference files on the current system (localhost) by using "file:///path/to/file"
+  if you are on Unix-based systems. If you're on Windows, use "file:///c:/path/to/file". In both
+  examples, note the three slashes (`/`) -- the first two slashes belong to `file://` and the third
+  one belongs to the absolute path to the file.
+
 
 Args:
   name: A unique name for this rule.