Ensure that all *.bak files are cleaned up

[jkeenan]

After 'make test' there are 9 '*.bak' files in the tree:

$ find . -type f -name '*.bak'
./t/000prereq.t.bak
./t/pod.t.bak
./t/meta-json.t.bak
./t/meta-yaml.t.bak
./t/db-threads.t.bak
./t/db-hash.t.bak
./t/db-recno.t.bak
./t/db-btree.t.bak
./DB_File.pm.bak

However, 'make clean' was only cleaning up 5 of them, leaving:

./t/000prereq.t.bak
./t/meta-json.t.bak
./t/meta-yaml.t.bak
./t/db-threads.t.bak

Better to use shell globs in the Makefile than hard-coding each name.

In the Perl 5 core distribution, cpan/DB_File/t/db-threads.t.bak was the only file not being cleaned up via 'make clean'.

[pmqs]

Thanks @jkeenan