CLN avoid some upcasting when its not the purpose of the test
Generally, tests follow the pattern:
- create input
- do something to it
- check that it matches expected output

There are a few places when, whilst creating the test input, some column is created of some dtype (e.g. 'int'), and then some incompatible type is set (e.g. a string `'foo'`), thus upcasting the column to dtype object. After that, this input is put through the function which the test is meant to test.

Given that the upcasting isn't the purpose of these tests, it'd be cleaner to just create the input right away with object dtype.

The "avoid upcasting to object" part of #50424 does seem uncontroversial, so this would help reduce the diff when a set of PRs to address that will be raised
good call, thanks