test: fix test failure in case of different default branch (#6857)

* test: fix test failure in case of different default branch

When the git config has set global `init.defaultBranch` different than master,
a few test cases were failing.
Currently, the test case is assuming the default branch to be set as
master.

This commit will introduce **initial_branch** option to git init call
which will ensure *master* branch is used as default for out test cases.

Set default git branch to main for test CI

Since main is going to be new default branch, it will let the test case
handle the scenario where default branch is set other than master.

Introduce isolation for git configuration

Co-authored-by: Saugat Pachhai (सौगात) <[email protected]>

.github/workflows/tests.yaml

@@ -67,10 +67,6 @@ jobs:
       run: |
         pip install --upgrade pip setuptools wheel
         pip install -e ".[dev]"
-    - name: setup git
-      run: |
-        git config --global user.email "[email protected]"
-        git config --global user.name "DVC Tester"
     - name: run tests
       timeout-minutes: 40
       run: >-