RCORE-2100: Restore ability to create test users

[nirinchev]

What, How & Why?

This is a follow-up on #7300, where we restore the ability for SDKs to construct a fake user for testing when actual sync session is not necessary. This is only intended to be used for tests and should never be exposed to users.

Fixes #7633.

☑️ ToDos

• 📝 Changelog update

[nirinchev]

The failing test doesn't seem to be related to this change.

[coveralls-official]

Pull Request Test Coverage Report for Build nikola.irinchev_476

Details

• 0 of 11 (0.0%) changed or added relevant lines in 1 file are covered.
• 942 unchanged lines in 59 files lost coverage.
• Overall coverage decreased (-0.002%) to 90.749%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/realm/object-store/sync/app.cpp	0	11	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/realm/sync/instructions.hpp	1	75.86%
src/realm/sync/network/default_socket.hpp	1	94.12%
src/realm/sync/noinst/server/crypto_server_apple.mm	1	66.28%
src/realm/sync/transform.cpp	1	61.05%
test/test_sync.cpp	1	93.11%
test/test_util_to_string.cpp	1	96.67%
src/realm/array_blob.hpp	2	85.71%
src/realm/object-store/impl/apple/external_commit_helper.cpp	2	74.77%
src/realm/object-store/sync/impl/apple/network_reachability_observer.cpp	2	65.57%
src/realm/owned_data.hpp	2	65.79%

Totals
Change from base Build 2262:	-0.002%
Covered Lines:	211964
Relevant Lines:	233573

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💛 - Coveralls