Add specs for Pointer::Appender (#14719)

spec/std/pointer/appender_spec.cr

@@ -0,0 +1,28 @@
+require "spec"
+
+describe Pointer::Appender do
+  it ".new" do
+    Pointer::Appender.new(Pointer(Void).null)
+  end
+
+  it "#<<" do
+    data = Slice(Int32).new(5)
+    appender = data.to_unsafe.appender
+    4.times do |i|
+      appender << (i + 1) * 2
+    end
+
+    data.should eq Slice[2, 4, 6, 8, 0]
+  end
+
+  it "#size" do
+    data = Slice(Int32).new(5)
+    appender = data.to_unsafe.appender
+    appender.size.should eq 0
+    4.times do |i|
+      appender << 0
+      appender.size.should eq i + 1
+    end
+    appender.size.should eq 4
+  end
+end