Exclude MRI tests failing due to missing regexp encoding flags in Prism

* And also some string/encoding-related missing SyntaxError from Prism.

Author: eregon

test/mri/excludes/TestM17N.rb

@@ -28,3 +28,11 @@
 exclude :test_sprintf_s, "needs investigation"
 exclude :test_string_inspect_encoding, "needs investigation"
 exclude :test_utf_dummy_are_like_regular_dummy_encodings, "<[0, 0, 254, 255]> expected but was"
+exclude :test_dynamic_eucjp_regexp, "prism missing regexp encoding flags"
+exclude :test_dynamic_sjis_regexp, "prism missing regexp encoding flags"
+exclude :test_dynamic_utf8_regexp, "prism missing regexp encoding flags"
+exclude :test_regexp_mixed_unicode, "prism missing regexp encoding flags"
+exclude :test_regexp_too_short_multibyte_character, "prism missing regexp encoding flags"
+exclude :test_regexp_unicode, "prism missing regexp encoding flags"
+exclude :test_regexp_usascii, "prism missing regexp encoding flags"
+exclude :test_string_mixed_unicode, "prism missing regexp encoding flags"

test/mri/excludes/TestMixedUnicodeEscape.rb

@@ -0,0 +1 @@
+exclude :test_basic, "prism missing regexp encoding flags"