csu tests: Remove extra slash

[jlduran]

Summary

Remove an extra slash that ends up in the metalog. This double slash
can produce an invalid specification file if there are subdirectories
down the hierarchy when sorted.

For example, consider the following metalog excerpt:

./base/aaa type=dir
./base//aaa/bbb type=dir

If sorted, would turn out:

./base//aaa/bbb type=dir
./base/aaa type=dir

Apparently missing the ./base/aaa directory in the specification.

Luckily here are no subdirectories.

[emaste]

We could add a check for double slash to tools/pkgbase/metalog_reader.lua also

[jlduran]

In D48778#1112946, @emaste wrote:

We could add a check for double slash to tools/pkgbase/metalog_reader.lua also

Yes! I forgot you mentioned it in another review. I'll take a look. Thank you.