Use ASCII-compatible encoding for tests

Since these files are written in a wide character encoding, stop at first NUL byte and are actually empty. ASCII-incompatible encodings have never been supported as source encoding.
2023-04-20 09:57:49 +09:00 · 2023-04-20 09:57:49 +09:00 · 7821708c84
--- a/spec/ruby/command_line/fixtures/freeze_flag_required_diff_enc.rb
+++ b/spec/ruby/command_line/fixtures/freeze_flag_required_diff_enc.rb
--- a/spec/ruby/language/fixtures/freeze_magic_comment_required_diff_enc.rb
+++ b/spec/ruby/language/fixtures/freeze_magic_comment_required_diff_enc.rb