[UT] Speed up BE unit test

[acelyc111]

Proposed changes

There are some long loops and sleeps in unit tests, they will cost a
very long time to complete all unit tests, especially run in TSAN mode.
This patch speed up unit tests by shortening long loops and sleeps,
on my environment all unit tests duration reduced from about 30
minutes to less than 1 minute. It's useful to do basic functional unit tests.
You can switch to run in this mode by adding a new environment variable
'DORIS_ALLOW_SLOW_TESTS'. For example, you can enable it by setting:
export DORIS_ALLOW_SLOW_TESTS=1
and also you can disable it by setting:
export DORIS_ALLOW_SLOW_TESTS=0

Types of changes

What types of changes does your code introduce to Doris?
Put an x in the boxes that apply

• [] Bugfix (non-breaking change which fixes an issue)
• [] New feature (non-breaking change which adds functionality)
• [] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [] Documentation Update (if none of the other choices apply)
• Code refactor (Modify the code structure, format the code, etc...)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• [] I have create an issue on (Fix #ISSUE), and have described the bug/feature there in detail
• Compiling and unit tests pass locally with my changes
• [] I have added tests that prove my fix is effective or that my feature works
• [] If this change need a document change, I have updated the document
• [] Any dependent changes have been merged

[kangkaisen]

+1