tests: force non-deterministic mode in non-deterministic tests

tests: force non-deterministic mode in non-deterministic tests

Since ar can be built defaulting to deterministic mode, tests which
expect non-deterministic behaviour need to explicitly set the U flag.

The non-deterministic member test expects SOURCE_DATE_EPOCH to not be
set; this documents that. Unconditionally unsetting the variable
causes issues in test infrastructure (which expects unsetenv to only
be called on variables which are already set).

Signed-off-by: Stephen Kitt <steve@sk2.org>