Add a test asserting that no plugin is an empty file

I'm still not completely sure how it happened, and I can't reproduce
it myself, but in the Debian build of ikiwiki 3.20180105, wikitext.pm
ended up empty. The build fixes in commits 3aacac3b, efcbeaa0,
b32480f0 hopefully fixed this.

Signed-off-by: Simon McVittie <smcv@debian.org>

diff --git a/t/not-truncated.t b/t/not-truncated.t
new file mode 100755 (executable)
index 0000000..15450c2
--- /dev/null
+++ b/t/not-truncated.t
@@ -0,0 +1,24 @@
+#!/usr/bin/perl
+# Regression test for a weird build issue where wikitext.pm was installed
+# as an empty file, possibly caused by parallel builds. This was hopefully
+# fixed by making the build steps more robust in 3.20180228.
+# https://buildd.debian.org/status/fetch.php?pkg=ikiwiki&arch=all&ver=3.20180105-1&stamp=1515285462&raw=0
+# -rw-r--r-- root/root         0 2018-01-06 23:20 ./usr/share/perl5/IkiWiki/Plugin/wikitext.pm
+use warnings;
+use strict;
+use Test::More;
+
+my @libs="IkiWiki.pm";
+push @libs, map { chomp; $_ } `find IkiWiki -type f -name \\*.pm`;
+
+plan(tests => scalar @libs);
+
+FILE: foreach my $file (@libs) {
+       foreach my $libdir (@INC) {
+               if (-e "$libdir/$file") {
+                       ok(-s "$libdir/$file", "$file available in $libdir and not truncated");
+                       next FILE;
+               }
+       }
+       fail("$file not available in ".join(':', @INC));
+}