Enable multi-byte tests on Linux

[marcelofabri]

This depends on https://bugs.swift.org/browse/SR-3448 and https://bugs.swift.org/browse/SR-3449 or finding a reliable workaround.

See #997 for more discussion.

[abl]

I've created swiftlang/swift-corelibs-foundation#796 in response to this; it should fix SR-3448 and SR-3449.

Also, as a temporary workaround, it is possible to use options: .regularExpression to do replacements across multi-byte substrings.

[marcelofabri]

Closed by #1563