tests: Explicitly unset SOURCE_DATE_EPOCH before running fs tests

tests: Explicitly unset SOURCE_DATE_EPOCH before running fs tests

In some filesystem utils like mksquashfs, they will silently change
behaviour and cause timestamps to unexpectedly change. Build
environments like Debian's set SOURCE_DATE_EPOCH in the environment,
so remove it. Reproducible builds are good and useful for shipped
artifacts, but this causes build-time tests to fail.

Signed-off-by: Steve McIntyre <steve@einval.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>