Manual partitions number setting in tests is broken

[amyskov]

System information

• OS Platform and Distribution (e.g., Linux Ubuntu 16.04): Ubuntu 20.04
• Modin version: d663730
• Python version: 3.8.6
• Code we can use to reproduce:

import os

os.environ["MODIN_ENGINE"] = "ray"

import modin.pandas as pd
pd.DEFAULT_NPARTITIONS = 1

data = [x for x in range(10000)]

df = pd.DataFrame(data)
print(len(df._query_compiler._modin_frame._partitions)) # <available number of cores>

Describe the problem

Occurred after 9a6695d commit.

Source code / logs

[amyskov]

Number of partitions can be controlled by setting MODIN_CPUS environment variable, but some tests (for example https://github.com/modin-project/modin/blob/master/modin/pandas/test/test_groupby.py#L31) still changes pd.DEFAULT_NPARTITIONS for partitions number setting.