Environment variables are used as a source for bootstrap variables anyway.
Consistently consider SVG_DISABLE_FONT_EMBEDDING to be enabled if the bootstrap
variable is present, regardless of its value.

No need to tset it in CppunitTest_sd_import_tests, as it only affects SvgExport.
(However, there is currently no test of svg export in
CppunitTests_sd_export_tests, either; it was last disabled with
92590776 "disable sd's regression test again"
and subsequently removed with f5dbe0ea "Usage of
XShapeDumper in Impress added.")

Change-Id: I3bc8e26953bd4b5a59f8230ee93fe99e0488d5cc