test/readline/test_readline.rb: Use TERM=vt100 to run the tests

Readline seems to emit an escape sequence `"\e[?1034h` at an
unpredictable timing when `TERM=xterm` or something.
This leads to a noisy random test failure.

http://rubyci.s3.amazonaws.com/amazon2/ruby-master/log/20211102T063005Z.fail.html.gz
```
  1) Failure:
TestReadline#test_modify_text_in_pre_input_hook [/home/chkbuild/chkbuild/tmp/build/20211102T063005Z/ruby/test/readline/test_readline.rb:404]:
<"> hello world\n"> expected but was
<"\e[?1034h> hello world\n">.
```

This test is for Ruby's readline extension, not for readline library
itself. So here this change force to use "TERM=vt100" to run the tests.

test/readline/test_readline.rb

@@ -7,6 +7,7 @@ require "open3"
 
 module BasetestReadline
   INPUTRC = "INPUTRC"
+  TERM = "TERM"
   SAVED_ENV = %w[COLUMNS LINES]
 
   TIMEOUT = 8
@@ -14,10 +15,12 @@ module BasetestReadline
   def setup
     @saved_env = ENV.values_at(*SAVED_ENV)
     @inputrc, ENV[INPUTRC] = ENV[INPUTRC], IO::NULL
+    @term, ENV[TERM] = ENV[TERM], "vt100"
   end
 
   def teardown
     ENV[INPUTRC] = @inputrc
+    ENV[TERM] = @term
     Readline.instance_variable_set("@completion_proc", nil)
     begin
       Readline.delete_text