test: Fix stalled tests on Ruby 3.1

[ashie]

Which issue(s) this PR fixes:
Fixes #3543

What this PR does / why we need it:
Several tests are stalled since Ruby 3.1, I found that it's caused by
the followig Ruby's change:

ruby/ruby@2d98593

It seems that there are some conditions to reproduce it:

• attr_reader is defined in a class
• $log is accessed by it
• assert is called with a block
• assert refers the $log via the attr_reader
• ... (some other conditions seem needed but not clarified yet)

Here is the sample test case to reproduce it:

require 'fluent/test'

class StallTest < Test::Unit::TestCase
  class Stall
    attr_reader :log
    def initialize
      @log = $log
    end
  end

  setup do
    Fluent::Test.setup
  end

  20.times do |i|
    test "stall #{i}" do
      assert { Stall.new.log }
    end
  end
end

Becasue all stalled tests don't need to use block, I remove these blocks
to avoid this issue.

Docs Changes:
None.

Release Note:
Same with the title.

[fujimotos]

I was initially confused by this patch, thinking "welp, isn't this patch
just removing spaces? Where's the actual diff?", and realized that the
fix is about converting {} to ().
Maybe I need to buy a pair of glasses.