Remove java.io.File dependency from shared tests

[libec]

Tests for the shared code can't run on iOS.

1. Check if it makes sense to remove java.io.File and use https://github.com/Kotlin/kotlinx-io instead.
2. Check if anyone runs KMP tests on iOS as mocking and the JVM environment might come in handy later

https://github.com/libec/ynab-kmm-api/blob/8a87ea89218d8de226e0c385f2b7928b4964a046/shared/src/commonTest/kotlin/mocks/HttpClientMockFactory.kt#L50-L56