test: set LD_LIBRARY_PATH early and keep its old contents

Previously LD_LIBRARY_PATH was exported (and environment changed)
in the middle of test case execution, when a function setting it
was called.

Previously the old contents of LD_LIBRARY_PATH was lost (if any)
when it was re-set and exported. In some systems the old contents of
LD_LIBRARY_PATH was needed to e.g. locate suitable gmime library.