[C++][FS][Azure] Make mutations of test data explicit in azurefs_test.cc

### Describe the enhancement requested

Some Filesystem tests need different initial state of the database so having `PreexisingContainerName()` and `PreexistingObjectName()` [1] as test globals prevent some test cases from having a clean slate starting point.

[1] https://github.com/apache/arrow/blob/main/cpp/src/arrow/filesystem/azurefs_test.cc#L216

### Component(s)

C++