[cookbook] remove temporary files created from scripts

Some scripts generate temporary files that shouldn't be cleaned up in the
script themselves (it wouldn't make sense in the context of the example).
Nevertheless, this leaves temporary files lying around on the filesystem
after running the test suite, thus the tests now clean these files up.

t/categories/cookbook/07file-access.t

@@ -14,6 +14,7 @@ subtest {
 
     my $output = run-example($example-name);
     is($output, $expected-output, $example-name);
+    unlink "test_file";  # the script generates this file
 }, "07-01opening_file.pl";
 
 #| run the given example script

t/categories/cookbook/14database-access.t

@@ -14,6 +14,7 @@ subtest {
 
     my $output = run-example($example-name);
     is($output, $expected-output, $example-name);
+    unlink "video.db"; # the script creates this file
 }, "14-09-dbi-sql.pl";
 
 #| run the given example script