GH-49364: [Ruby] Simplify reader tests (#49365)

### Rationale for this change

The current reader tests use a sub test case per type but we can use a test per type like the current writer tests.

### What changes are included in this PR?

* Create test data in a test not a setup to use a test no a sub test case per type
* Add `ArrowFormat::Type#to_s`
* Add `ArrowFormat::FileReader#schema`

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #49364

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

Author: kou

ruby/red-arrow-format/lib/arrow-format/file-reader.rb

@@ -34,6 +34,7 @@ class FileReader
     FOOTER_SIZE_FORMAT = :s32
     FOOTER_SIZE_SIZE = IO::Buffer.size_of(FOOTER_SIZE_FORMAT)
 
+    attr_reader :schema
     def initialize(input)
       case input
       when IO

ruby/red-arrow-format/lib/arrow-format/type.rb

@@ -16,6 +16,9 @@
 
 module ArrowFormat
   class Type
+    def to_s
+      name
+    end
   end
 
   class NullType < Type
@@ -412,6 +415,10 @@ def initialize(bit_width, unit)
       @unit = unit
     end
 
+    def to_s
+      "#{super}(#{unit})"
+    end
+
     def to_flatbuffers
       fb_type = FB::Time::Data.new
       fb_type.bit_width = @bit_width
@@ -477,6 +484,12 @@ def build_array(size, validity_buffer, values_buffer)
       TimestampArray.new(self, size, validity_buffer, values_buffer)
     end
 
+    def to_s
+      options = [@unit]
+      options << @time_zone if @time_zone
+      "#{super}(#{options.join(", ")})"
+    end
+
     def to_flatbuffers
       fb_type = FB::Timestamp::Data.new
       fb_type.unit = FB::TimeUnit.try_convert(@unit.to_s.upcase)
@@ -581,6 +594,10 @@ def build_array(size, validity_buffer, values_buffer)
       DurationArray.new(self, size, validity_buffer, values_buffer)
     end
 
+    def to_s
+      "#{super}(#{@unit})"
+    end
+
     def to_flatbuffers
       fb_type = FB::Duration::Data.new
       fb_type.unit = FB::TimeUnit.try_convert(@unit.to_s.upcase)
@@ -730,6 +747,10 @@ def build_array(size, validity_buffer, values_buffer)
       FixedSizeBinaryArray.new(self, size, validity_buffer, values_buffer)
     end
 
+    def to_s
+      "#{super}(#{@byte_width})"
+    end
+
     def to_flatbuffers
       fb_type = FB::FixedSizeBinary::Data.new
       fb_type.byte_width = @byte_width
@@ -746,6 +767,10 @@ def initialize(byte_width, precision, scale)
       @scale = scale
     end
 
+    def to_s
+      "#{name}(#{@precision}, #{@scale})"
+    end
+
     def to_flatbuffers
       fb_type = FB::Decimal::Data.new
       fb_type.bit_width = @byte_width * 8
@@ -789,6 +814,10 @@ def initialize(child)
       super()
       @child = child
     end
+
+    def to_s
+      "#{super}<#{child.name}: #{child.type}>"
+    end
   end
 
   class ListType < VariableSizeListType
@@ -842,6 +871,13 @@ def build_array(size, validity_buffer, children)
       StructArray.new(self, size, validity_buffer, children)
     end
 
+    def to_s
+      fields = children.collect do |child|
+        "#{child.name}: #{child.type}"
+      end
+      "#{super}<#{fields.join(", ")}>"
+    end
+
     def to_flatbuffers
       FB::Struct::Data.new
     end
@@ -880,6 +916,11 @@ def build_array(size, validity_buffer, offsets_buffer, child)
       MapArray.new(self, size, validity_buffer, offsets_buffer, child)
     end
 
+    def to_s
+      key, value, = child.type.children
+      "#{name}<#{key.type}, #{value.type}>"
+    end
+
     def to_flatbuffers
       FB::Map::Data.new
     end
@@ -900,6 +941,13 @@ def resolve_type_index(type)
       @type_indexes[type] ||= @type_ids.index(type)
     end
 
+    def to_s
+      children = @children.collect.with_index do |child, i|
+        "#{child.name}: #{child.type}=#{@type_ids[i]}"
+      end
+      "#{super}<#{children.join(", ")}>"
+    end
+
     def to_flatbuffers
       fb_type = FB::Union::Data.new
       fb_type.mode = FB::UnionMode.try_convert(@mode.to_s.capitalize)
@@ -975,5 +1023,10 @@ def build_fb_field(fb_field, field)
       fb_field.type = @value_type.to_flatbuffers
       fb_field.dictionary = fb_dictionary_encoding
     end
+
+    def to_s
+      "#{super}<index=#{@index_type}, value=#{@value_type}, " +
+        "ordered=#{@ordered}>"
+    end
   end
 end