[lldb] Make TestDataFormatterObjCCF.py pass outside California

Summary:
This test creates its dates with `NSDate dateWithNaturalLanguageString` which is deprecated and uses the current time zone of the machine to
interpret the input string. This causes that the created NSDate has a different value depending on the locale of the machine
and we hardcoded the value for California's time zone (PST) but the data formatter gives out the GMT value as a string.

This just replaces the use with the timezone-independent dateWithTimeIntervalSince1970 (which we also use in the rest of the test)
to make this pass independently of the time zone of the machine running the test.

Reviewers: mib

Reviewed By: mib

Subscribers: lldb-commits, JDevlieghere

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D74038