Use C locale in UITest

b96180cb "uitest: set en_US.UTF8 for the
LibreOffice instance" had made UITest use the en_US.UTF-8 locale (passed from
LIBO_LANG to LC_ALL when starting soffice in uitest/libreoffice/connection.py),
for unstated reasons.

The mail sub-thread starting at
<https://lists.freedesktop.org/archives/libreoffice/2018-October/081318.html>
"Re: master build problems with en_US.utf8 locale" now argues that support for
an en_US.UTF-8 locale in the OS should not be necessary when building LO.  While
absence of OS-support for en_US.UTF-8 apparently doesn't break the UITests (see
<https://lists.freedesktop.org/archives/libreoffice/2018-November/081375.html>
"Re: master build problems with en_US.utf8 locale"), it feels better to make
these tests not use the en_US.UTF-8 locale at all.  At least for me, the tests
ran fine when using the C locale instead.

Change-Id: I23eb2ce540bb40a7b7d13c2a396e313966f03f6e
Reviewed-on: https://gerrit.libreoffice.org/63360
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

solenv/gbuild/UITest.mk

@@ -37,7 +37,7 @@ else
 gb_UITest_SOFFICEARG:=path:$(INSTROOT)/$(LIBO_BIN_FOLDER)/soffice
 endif
 
-gb_UITest_COMMAND = LIBO_LANG=en_US.UTF-8 $(gb_CppunitTest_RR) $(gb_UITest_EXECUTABLE) $(SRCDIR)/uitest/test_main.py
+gb_UITest_COMMAND = LIBO_LANG=C $(gb_CppunitTest_RR) $(gb_UITest_EXECUTABLE) $(SRCDIR)/uitest/test_main.py
 
 .PHONY : $(call gb_UITest_get_clean_target,%)
 $(call gb_UITest_get_clean_target,%) :