Build: Unit test ScFiltersTest::testUnicodeFileNameGnumeric() fails when LANG=en_US.iso88591

Hello,

when building current master in Ubuntu 16.04 in a console 
where LANG is set to US.iso88591 the unit test
ScFiltersTest::testUnicodeFileNameGnumeric()
in sc/qa/unit/subsequent_filters-test.cxx fails with the
following assertion:

===

assertion failed
- Expression: xDocSh.is()

ScFiltersTest::testUnicodeFileNameGnumeric finished in: 10ms
subsequent_filters-test.cxx:3937:Assertion
Test name: ScFiltersTest::testUnicodeFileNameGnumeric
assertion failed
- Expression: xDocSh.is()

===

The test passes when setting LANG to en_US.utf8.

I cannot judge whether this is acceptable behavior - today
it is IMHO not very common to set LANG to something other
than utf8.

Best regards

-Ulrich

-- 
Ulrich Gemkow
Institute of Communication Networks and Computer Engineering (IKR)
University of Stuttgart, Germany