Rails 6.1: Skip tests on Windows because of atomic write issue

[aidanharan]

Coercing tests on Windows that rely on SchemaCache#dump_to. The call in https://github.com/rails/rails/blob/e2781c2d93808031fbc4dbecaa7aac59aa90b4b8/activesupport/lib/active_support/core_ext/file/atomic.rb#L50 to rename the file fails on Windows. This is a Rails/Windows issue.

MacOS

> tempfile = Tempfile.new('test.txt')
=> #<File:/var/folders/mp/_9w8583s18z93h_w2s7zxq24ks97j0/T/test.txt20210429-16257-39oo5i>
> File.atomic_write(tempfile.path) { |file| file.write('BLAH') }
=> 4

Windows

> tempfile = Tempfile.new('test.txt')
=> #<Tempfile:C:/Users/user/AppData/Local/Temp/test.txt20210429-8740-1pwkymo>
> File.atomic_write(tempfile.path) { |file| file.write('BLAH') }
*** Errno::EACCES Exception: Permission denied @ rb_file_s_rename - (C:/Users/user/AppData/Local/Temp/.test.txt20210429-8740-1pwkymo20210429-8740-yjlmty, C:/Users/user/AppData/Local/Temp/test.txt20210429-8740-1pwkymo)

An example of exception raised is (https://ci.appveyor.com/project/wpolicarpo/activerecord-sqlserver-adapter/builds/38929321#L397):

ActiveRecord::ConnectionAdapters::SchemaCacheTest#test_marshal_dump_and_load_with_gzip_coerced:
Errno::EACCES: Permission denied @ rb_file_s_rename - (C:/Users/appveyor/AppData/Local/Temp/1/.schema_cache-20210429-3648-11o6yfo.dump.gz20210429-3648-4s5ga8, C:/Users/appveyor/AppData/Local/Temp/1/schema_cache-20210429-3648-11o6yfo.dump.gz)
    rails (8b63ea762239) activesupport/lib/active_support/core_ext/file/atomic.rb:50:in `rename'
    rails (8b63ea762239) activesupport/lib/active_support/core_ext/file/atomic.rb:50:in `block in atomic_write'
    C:/Ruby26-x64/lib/ruby/2.6.0/tempfile.rb:295:in `open'
    rails (8b63ea762239) activesupport/lib/active_support/core_ext/file/atomic.rb:24:in `atomic_write'
    rails (8b63ea762239) activerecord/lib/active_record/connection_adapters/schema_cache.rb:211:in `open'
    rails (8b63ea762239) activerecord/lib/active_record/connection_adapters/schema_cache.rb:158:in `dump_to'

Also in this PR, no longer need to coerce the DatabaseTasksDumpSchemaCacheTest#test_dump_schema_cache test as it has been updated on the Rails test suite and will now work on Windows.