Reorganize zcl_excel_common unit tests, one method per test and define several test classes instead of one

[sandraros]

Currently zcl_excel_common has one big test class with around 80 test methods.

Some methods like excel_string_to_date do several tests. Better define one method per test.

It's difficult to maintain.

Reorganizing may possibly help to troubleshoot transpiler issues (like https://github.com/abaplint/transpiler/issues/1431).