Namespace temporary directories for test cases

We avoid any name clashes during the test suite run. And the hierarchy
gets more clear when debugging in case any problem arises.

scripts/t/500_Dpkg_Path.t

@@ -22,8 +22,9 @@ use_ok('Dpkg::Path', 'canonpath', 'resolve_symlink',
        'check_files_are_the_same', 'get_pkg_root_dir',
        'guess_pkg_root_dir', 'relative_to_pkg_root');
 
-my $tmpdir = 't.tmp';
+my $tmpdir = 't.tmp/500_Dpkg_Path';
 
+mkdir $tmpdir;
 mkdir "$tmpdir/a";
 mkdir "$tmpdir/a/b";
 mkdir "$tmpdir/a/b/c";

scripts/t/900_update_alternatives.t

@@ -21,7 +21,7 @@ use strict;
 use warnings;
 
 my $srcdir = $ENV{srcdir} || '.';
-my $tmpdir = 't.tmp';
+my $tmpdir = 't.tmp/900_update_alternatives';
 my $admindir = File::Spec->rel2abs("$tmpdir/admindir"),
 my $altdir = File::Spec->rel2abs("$tmpdir/alternatives");
 my $bindir = File::Spec->rel2abs("$tmpdir/bin");