test-bot: use IO#read instead of IO#gets

IO#gets will only return for every new lines, which will be bad for
`brew install` since we print dots in single line.

Closes Homebrew/homebrew#44264.

Signed-off-by: Xu Cheng <xucheng@me.com>

Library/Homebrew/cmd/test-bot.rb

@@ -154,9 +154,12 @@ module Homebrew
           working_dir.cd { exec(*@command) }
         end
         write.close
-        while line = read.gets
-          puts line if verbose
-          @output += line
+        while buf = read.read(1)
+          if verbose
+            print buf
+            $stdout.flush
+          end
+          @output << buf
         end
       ensure
         read.close