windows: re-enable TestWinVerifyTrust with newly signed file

Rather than disabling this test, let's just not make it rely on
Microsoft files, whose signing validity period we can't depend on.
Instead, we include our own EV-signed artifact, with a Digicert
timestamp using a certificate valid for a decade.

Fixes golang/go#49651.
Fixes golang/go#49266.
For golang/go#46906.

Change-Id: Idadba346810017b8f769d6fac1ddd357d4dee93c
Reviewed-on: https://go-review.googlesource.com/c/sys/+/366655
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

windows/testdata/README

@@ -0,0 +1,21 @@
+This folder contains various pre-generated artifacts for testing. Descriptions
+of each follow below.
+
+## ev-signed-file.exe
+
+This was generated with:
+
+  int main(void)
+  {
+      puts("Hello Gophers!");
+      return 0;
+  }
+
+And then a simple clang/mingw compilation:
+
+  i686-w64-mingw32-gcc -Os -s a.c
+
+After, it was copied to a Windows computer where it was signed with an EV
+certificate using:
+
+  signtool sign /sha1 <ID of certificate> /fd sha256 /tr http://timestamp.digicert.com /td sha256 /d "Go Project EV Signing Test" a.exe