Fix `date --date` dependent test

This test depends on a nonportable option for date(1). I am not sure
that there is a good portable way to do this, so I just directly
included the seconds since epoch that it would have computed. We're
testing scdoc, not testing the local system's date command.

test/preamble

@@ -70,8 +70,7 @@ test-manual(8) "" "Header"
 EOF
 end 0
 
-export TZ=UTC
-export SOURCE_DATE_EPOCH=$(date --date="2017-12-09 23:18:57" +'%s')
+export SOURCE_DATE_EPOCH=1512861537
 
 begin "Supports \$SOURCE_DATE_EPOCH"
 scdoc <<EOF | grep '^\.TH "reproducible-manual" "8" "2017-12-09"' >/dev/null