This test started failing when updating to ICU 64, because ICU supports "zh"
and "zh-Hans-CN", but not explicitly also "zh-Hans", which is required for this
test to pass. The same kind of error is reproducible with ICU <64 when "Guru"
is added to the list of script codes in 'testIntl.js', because ICU supports
"pa-Guru-IN", but "pa-IN" isn't explicitly supported, too.
So, change this test to also check 'byFallback' to see if a locale is supported.
Drive-by change:
- Modernise the test to make it more readable how subtags are combined.
- Also add "419" to the list of region codes to cover the digit region syntax.
André Bargull