Remove seconds from test_get_gmt_from_date_string_date()

[SergeyBiryukov]

test_get_gmt_from_date_string_date() causes occasional random failures if a second has passed between strtotime( 'now' ) and get_gmt_from_date() calls:

1) Tests_Formatting_Date::test_get_gmt_from_date_string_date
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'2017-10-18 18:18:12'
+'2017-10-18 18:18:13'

I'd suggest removing seconds from this test, as it doesn't need that kind of precision, it just needs to make sure the timezone is correct.

[SergeyBiryukov]

Introduced in [35284].

[SergeyBiryukov]

Duplicate of #38815.