build: Cleanup dpkg-divert unit-test environment

Remove variables set by dpkg which affect the dpkg-divert behaviour,
and as such make it unreliable and cause test failures.

Closes: #687656

debian/changelog

@@ -12,6 +12,8 @@ dpkg (1.16.9) UNRELEASED; urgency=low
     Closes: #686995
   * Comment out dpkg(1) documentation about disabled --command-fd option.
     Closes: #685677
+  * Cleanup dpkg-divert unit-test environment to avoid build failures.
+    Closes: #687656
 
   [ Updated programs translations ]
   * Czech (Miroslav Kure).

src/t/100_dpkg_divert.t

@@ -20,6 +20,10 @@ use Dpkg::IPC;
 use strict;
 use warnings;
 
+# Cleanup environment from variables that pollute the test runs.
+delete $ENV{DPKG_MAINTSCRIPT_PACKAGE};
+delete $ENV{DPKG_MAINTSCRIPT_ARCH};
+
 my $srcdir = $ENV{srcdir} || '.';
 my $builddir = $ENV{builddir} || '.';
 my $tmpdir = 't.tmp/950_dpkg_divert';