Get rid of hardcoded timezone in tests

[dtaimanov]

Problem

Some tests failed in the case of building Jmix on a computer with a timezone different from +04. E.g. io.jmix.graphql.datafetcher.date.DateMutationTest#"should upsert time fields with seconds". It happens because there are tests with hardcoded timezone, e.g. 12:21:10+04:00.

Solution

Rework such tests to not use hardcoded timezone.

Please, check tests:

• io.jmix.graphql.datafetcher.date.DateFilterTest
• io.jmix.graphql.datafetcher.date.DateMutationTest
• io.jmix.graphql.datafetcher.date.DateQueryTest
• io.jmix.graphql.schema.scalar.OffsetDateTimeScalarTest
• io.jmix.graphql.schema.scalar.OffsetTimeScalarTest

Check also places where +04 appears in string literals.