Function summarize_categorical test suite

[HrayrMuradyan]

Plan the summarize_categorical function test suite.

[stephqwu]

Use cases

1. Dataframe has categorical variables with at least one non-null type
2. Dataframe has one categorical variable with at least one non-null type
3. Second argument is one of the accepted enumeration

Edge cases

1. Dataframe has no categorical variables
2. Dataframe has a categorical variable with all null values
3. Too few data to create plot type

Erroneous/adversarial input

1. First argument is not a dataframe
2. Second argument is not left as default or an accepted enumeration