Adapt / remove unit tests of private methods

[seddonym]

We're going to move to a Rust-based ImportScanner class - we don't want to have to expose this private method. This moves the tests for non-ascii based scanning to a higher level test, and removes the rest. There are already lower level Rust tests.

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #223 will improve performances by 16.82%

_{Comparing unit-tests-import-scanner-no-private-methods (4b02c7c) with master (ce0d32f)}

Summary

⚡ 3 improvements
✅ 19 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_deep_layers_large_graph_kept	19.8 ms	16.9 ms	+16.82%
⚡	test_no_chain	1.2 ms	1.1 ms	+12.35%
⚡	test_no_chains	1.2 ms	1.1 ms	+12.35%