ibm866 encoding label appears incorrect in tests

[ChALkeR]

workerd/src/workerd/api/tests/encoding-test.js

Lines 404 to 407 in b026653

	['866', 'ibm-866'],
	['cp866', 'ibm-866'],
	['csibm866', 'ibm-866'],
	['ibm866', 'ibm-866'],

new TextDecoder('866').encoding should instead be ibm866, not ibm-866

See https://encoding.spec.whatwg.org/#names-and-labels (and https://encoding.spec.whatwg.org/encodings.json)

Likewise, new TextDecoder('shift-jis').encoding should be shift_jis, not shift-jis:

workerd/src/workerd/api/tests/encoding-test.js

Lines 592 to 599 in b026653

	['csshiftjis', 'shift-jis'],
	['ms932', 'shift-jis'],
	['ms_kanji', 'shift-jis'],
	['shift-jis', 'shift-jis'],
	['shift_jis', 'shift-jis'],
	['sjis', 'shift-jis'],
	['windows-31j', 'shift-jis'],
	['x-sjis', 'shift-jis'],