test: Correct setexpr test prefix

test: Correct setexpr test prefix

This prefix should be for setexpr, not mem. This means that trying to
select just these tests to run does not work. Fix it.

For some reason this provokes an assertion failure due to memory not
being freed. Move the env_set() in setexpr_test_str() to before the
malloc() heap size size is recorded and disable the rest in
setexpr_test_str_oper().

Signed-off-by: Simon Glass <sjg@chromium.org>