tests: Unset SOURCE_DATE_EPOCH

Some distributions set this during build in order to have reproducible
builds from the same source code: for example, Debian uses the date
from debian/changelog.

However, some of our tests assume that `ostree commit` will result in
a commit with the current date/time, and SOURCE_DATE_EPOCH breaks that
assumption. Unset it for our build-time tests.

Bug: ostreedev/ostree#2405
Signed-off-by: Simon McVittie <smcv@collabora.com>
Applied-upstream: 2021.4, commit:2c5fa2cdb6c304394ca629c959ec53d878f3f93a

Gbp-Pq: Name tests-Unset-SOURCE_DATE_EPOCH.patch

tests/libtest.sh

@@ -69,6 +69,10 @@ if ! test -f .testtmp; then
     touch .testtmp
 fi
 
+# Some distribution builds set this, but some of our build-time tests
+# assume this won't be used when committing
+unset SOURCE_DATE_EPOCH
+
 # Also, unbreak `tar` inside `make check`...Automake will inject
 # TAR_OPTIONS: --owner=0 --group=0 --numeric-owner presumably so that
 # tarballs are predictable, except we don't want this in our tests.