Add instructions to codegen test failures

[stroxler]

Summary

It's nice that the tests warn us if you need to rerun code generation, but we don't actually document how to run code generation anywhere so every time I hit the failures I get frustrated trying to remember!

I thought about adding it to the README but I'm not actually sure it adds any value - instead, why not add it to the relevant failures. That way the codegen instructions aren't adding noise to the general getting-started docs, but they are right there whenever we need them.

Test Plan

In a branch where I added lpar and rpar to With statements, I ran

> python -m unittest libcst.codegen.tests.test_codegen_clean
FFF
======================================================================
FAIL: test_codegen_clean_matcher_classes (libcst.codegen.tests.test_codegen_clean.TestCodegenClean)
Verifies that codegen of matcher classes would not result in a
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/stroxler/kode/LibCST/libcst/codegen/tests/test_codegen_clean.py", line 95, in test_codegen_clean_matcher_classes
    self.assert_code_matches(old_code, new_code, "libcst.matchers.__init__")
  File "/Users/stroxler/kode/LibCST/libcst/codegen/tests/test_codegen_clean.py", line 25, in assert_code_matches
    self.assertTrue(
AssertionError: False is not true : libcst.matchers.__init__ needs new codegen, see `python -m libcst.codegen.generate --help` for instructions

======================================================================
FAIL: test_codegen_clean_return_types (libcst.codegen.tests.test_codegen_clean.TestCodegenClean)
Verifies that codegen of return types would not result in a
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/stroxler/kode/LibCST/libcst/codegen/tests/test_codegen_clean.py", line 128, in test_codegen_clean_return_types
    self.assert_code_matches(old_code, new_code, "libcst.matchers._return_types")
  File "/Users/stroxler/kode/LibCST/libcst/codegen/tests/test_codegen_clean.py", line 25, in assert_code_matches
    self.assertTrue(
AssertionError: False is not true : libcst.matchers._return_types needs new codegen, see `python -m libcst.codegen.generate --help` for instructions

======================================================================
FAIL: test_codegen_clean_visitor_functions (libcst.codegen.tests.test_codegen_clean.TestCodegenClean)
Verifies that codegen of visitor functions would not result in a
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/stroxler/kode/LibCST/libcst/codegen/tests/test_codegen_clean.py", line 63, in test_codegen_clean_visitor_functions
    self.assert_code_matches(old_code, new_code, "libcst._typed_visitor")
  File "/Users/stroxler/kode/LibCST/libcst/codegen/tests/test_codegen_clean.py", line 25, in assert_code_matches
    self.assertTrue(
AssertionError: False is not true : libcst._typed_visitor needs new codegen, see `python -m libcst.codegen.generate --help` for instructions

----------------------------------------------------------------------
Ran 3 tests in 40.231s

FAILED (failures=3)

[zsol]

I normally run tox -e codegen if I'm not in a hurry, I'd recommend that to new folks. Great idea btw!

[codecov-commenter]

Codecov Report

Merging #582 (02e82df) into main (d9a1dc8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #582   +/-   ##
=======================================
  Coverage   94.73%   94.73%           
=======================================
  Files         241      241           
  Lines       24658    24660    +2     
=======================================
+ Hits        23360    23362    +2     
  Misses       1298     1298           

Impacted Files	Coverage Δ
libcst/codegen/tests/test_codegen_clean.py	89.09% <100.00%> (+0.41%)	⬆️

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