related fdo#63230: Fix unit test for fdo#44286 to run in all locales

fdo#44286 test fails depending on system language. To fix it, this commit sets
output language to LANGUAGE_ENGLISH_US.

Erorr message:
| $ LANG=fr_LU.UTF-8 make -rs CppunitTest_svl_qa_cppunit
| [build CUT] svl_qa_cppunit
| /home/master/src/libreoffice/workdirs/master/svl/qa/unit/svl.cxx:403:(anonymous
| namespace)::Test::testFdo44286
| equality assertion failed
| - Expected: 1902-04-22
| - Actual  : YYYY-04-DD
|
| svl.cxx:403:Assertion
| Test name: (anonymous namespace)::Test::testFdo44286
| equality assertion failed
| - Expected: 1902-04-22
| - Actual  : YYYY-04-DD
|
| Failures !!!
| Run: 7   Failure total: 1   Failures: 1   Errors: 0

Change-Id: I58756c7508c7f02bec9c3b7b693032265d18c6ae
Reviewed-on: https://gerrit.libreoffice.org/9826
Reviewed-by: Lionel Elie Mamane <lionel@mamane.lu>
Tested-by: Lionel Elie Mamane <lionel@mamane.lu>