maximum showing line number on btest failure

Some tests in btest uses long src for btest and it is harmful to
check the results. This patch introducing the limitation how many
lines of code is shown on failure.

bootstraptest/runner.rb

@@ -749,6 +749,8 @@ end
 
 def pretty(src, desc, result)
   src = src.sub(/\A\s*\n/, '')
+  lines = src.lines
+  src = lines[0..20].join + "(...snip)\n" if lines.size > 20
   (/\n/ =~ src ? "\n#{adjust_indent(src)}" : src) + "  #=> #{desc}"
 end