Change handling of qpdf fuzz corpus

Files are copied into the build area rather than left in the source
tree, and the test suite looks for them there. Also remove special
case around counting files in the qpdf corpus.

README-maintainer

@@ -66,9 +66,10 @@ GOOGLE OSS-FUZZ
 * qpdf project: https://github.com/google/oss-fuzz/tree/master/projects/qpdf
 
 * Adding new test cases: download the file from oss-fuzz and drop it
-  in fuzz/qpdf_extra/issue-number.fuzz. If not ready to include, it
-  can be stored anywhere, and the absolute path can be passed to the
-  reproduction code as described below.
+  in fuzz/qpdf_extra/issue-number.fuzz. When ready to include it, add
+  to fuzz/CMakeLists.txt. Until ready to use, the file can be stored
+  anywhere, and the absolute path can be passed to the reproduction
+  code as described below.
 
 * To test locally, see https://github.com/google/oss-fuzz/tree/master/docs/,
   especially new_project_guide.md. Summary:

fuzz/qtest/fuzz.test

@@ -9,10 +9,7 @@ require TestDriver;
 
 my $td = new TestDriver('fuzz');
 
-my $qpdf_n_test_files = 31;
-my @extra = glob("../qpdf_extra/*.fuzz");
-my $qpdf_n_extra_files = scalar(@extra);
-my $qpdf_n_files = ($qpdf_n_test_files + $qpdf_n_extra_files);
+my $qpdf_corpus = $ENV{'QPDF_FUZZ_CORPUS'} || die "must set QPDF_FUZZ_CORPUS";
 
 my @fuzzers = (
     ['ascii85' => 1],
@@ -23,7 +20,7 @@ my @fuzzers = (
     ['pngpredictor' => 1],
     ['runlength' => 6],
     ['tiffpredictor' => 1],
-    ['qpdf' => $qpdf_n_files],
+    ['qpdf' => 52],             # increment when adding new files
     );
 
 my $n_tests = 0;
@@ -40,7 +37,7 @@ foreach my $d (@fuzzers)
     my $dir = "../${k}_fuzzer_seed_corpus";
     if (! -d $dir)
     {
-        $dir = "../build/${k}_fuzzer_seed_corpus";
+        $dir = $qpdf_corpus;
     }
     my @files = glob("$dir/*");
     $td->runtest("file count for $dir",