Prohibit copying to `fixtures` and dot-direcotry just under `test`

Do not add directories which would let chkbuild fires accidentally, to
just under `test`.  The "fixtures" directory is often used to place
test stuffs.

tool/sync_default_gems.rb

@@ -600,7 +600,7 @@ module SyncDefaultGems
         puts "Remove files added to toplevel: #{toplevels.join(', ')}"
         system(*%w"git rm -r --", *toplevels)
       end
-      tools = changed.select {|f|f.start_with?("test/lib/", "tool/")}
+      tools = changed.select {|f|f.start_with?("test/fixtures/", "test/lib/", "tool/")}
       unless tools.empty?
         system(*%W"git rm -r --", *tools)
         system(*%W"git checkout HEAD~ --", *tools)

tool/test/test_sync_default_gems.rb

@@ -96,7 +96,9 @@ module Test_SyncDefaultGems
         if dir == "src"
           Dir.mkdir("#{dir}/lib")
           File.write("#{dir}/lib/fine.rb", "return\n")
-          system(*%W"git add lib/fine.rb", exception: true, chdir: dir)
+          Dir.mkdir("#{dir}/test")
+          File.write("#{dir}/test/test_fine.rb", "return\n")
+          system(*%W"git add lib/fine.rb test/test_fine.rb", exception: true, chdir: dir)
           system(*%W"git commit -q -m", "Looks fine", exception: true, chdir: dir)
         end
         Dir.mkdir("#{dir}/tool")
@@ -154,6 +156,20 @@ module Test_SyncDefaultGems
       assert_equal(@sha["src"], IO.popen(%W[git log --format=%H -1], chdir: "src", &:read).chomp, out)
     end
 
+    def test_skip_test_fixtures
+      Dir.mkdir("#@target/test")
+      Dir.mkdir("#@target/test/fixtures")
+      File.write("#@target/test/fixtures/fixme.rb", "")
+      system(*%W"git add test/fixtures/fixme.rb", exception: true, chdir: @target)
+      system(*%W"git commit -q -m", "Add fitures", exception: true, chdir: @target)
+      out = capture_process_output_to([STDOUT, STDERR]) do
+        Dir.chdir("src") do
+          SyncDefaultGems.sync_default_gems_with_commits(@target, ["#{@sha[@target]}..#{@target}/default"])
+        end
+      end
+      assert_equal(@sha["src"], IO.popen(%W[git log --format=%H -1], chdir: "src", &:read).chomp, out)
+    end
+
     def test_skip_toplevel
       Dir.mkdir("#@target/docs")
       File.write("#@target/docs/NEWS.md", "= NEWS!!!\n")