(PUP-12047) Add test to ensure MD5 checksum is skipped for FIPS

[AriaXLi]

This commit adds a test to http_metadata_spec.rb that checks that the MD5 checksum type is skipped in the collect method on FIPS enabled platforms.

[AriaXLi]

This PR adds a test to the changes made in #9405

[AriaXLi]

This test will fail & not skip md5 when I revert 1f7c8ef:

❯ git revert -m1 1f7c8ef
❯ bundle exec rspec ./spec/unit/file_serving/http_metadata_spec.rb:38
Run options:
  include {:locations=>{"./spec/unit/file_serving/http_metadata_spec.rb"=>[38]}}
  exclude {:benchmark=>true}
F

Failures:

  1) Puppet::FileServing::HttpMetadata when initializing skips md5 checksum type in collect on FIPS enabled platforms
     Failure/Error: expect( metadata.checksum_type ).to eq :mtime

       expected: :mtime
            got: :md5

       (compared using ==)

       Diff:
       @@ -1 +1 @@
       -:mtime
       +:md5

     # ./spec/unit/file_serving/http_metadata_spec.rb:37:in `block (3 levels) in <top (required)>'
     # ./spec/spec_helper.rb:180:in `block (2 levels) in <top (required)>'