const testing.markErrBegin

4 uses

	testing (current package)
		testing.go#L567: 	markErrBegin byte = 'O' &^ '@' // ^O: start of error
		testing.go#L679: 		case markFraming, markErrBegin, markErrEnd, markEscape:
		testing.go#L1270: 		strErrBegin = string(markErrBegin)
		testing.go#L1320: 		case markFraming, markEscape, markErrBegin, markErrEnd:

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