debian: Mark libio-string-perl as <!nocheck>

This allows building the package without running the test suite, and
not requiring build dependencies specific to the test suite. Which
makes it easier to bootstrap an architecture.

Prompted-by: Helmut Grohne <helmut@subdivi.de>

debian/changelog

@@ -58,6 +58,7 @@ dpkg (1.18.2) UNRELEASED; urgency=low
       options first.
   * Packaging:
     - Make the libdpkg-dev package Multi-Arch:same.
+    - Mark libio-string-perl as <!nocheck>.
   * Documentation:
     - Fix grammar in dpkg-architecture(1).
       Thanks to Chris Lamb <lamby@debian.org>. Closes: #787616

debian/control

@@ -15,7 +15,7 @@ Build-Depends: debhelper (>= 9), pkg-config, flex,
  libselinux1-dev (>= 1.28-4) [linux-any],
  libkvm-dev [kfreebsd-any],
  libncursesw5-dev,
- libio-string-perl
+ libio-string-perl <!nocheck>
 
 Package: libdpkg-dev
 Section: libdevel