t0050: use explicit branch name and verify checkout result

The test 'checkout with no pathspec and a case insensitive fs'
relies on the script-wide GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
to set the initial branch to 'main'.  Use 'git init -b main'
instead, so that the test is self-contained and does not depend
on the blanket setting for the entire script.

While at it, add a test_path_is_file check after the checkout
to verify that the expected file is present in the worktree.

Signed-off-by: Md Ferdous Alam <mdferdousalam1989@yahoo.com>

Author: mdferdousalam

t/t0050-filesystem.sh

@@ -118,7 +118,7 @@ $test_unicode 'merge (silent unicode normalization)' '
 '
 
 test_expect_success CASE_INSENSITIVE_FS 'checkout with no pathspec and a case insensitive fs' '
-	git init repo &&
+	git init -b main repo &&
 	(
 		cd repo &&
 
@@ -133,7 +133,8 @@ test_expect_success CASE_INSENSITIVE_FS 'checkout with no pathspec and a case in
 		git add gitweb &&
 		git commit -m "add gitweb/subdir/file" &&
 
-		git checkout main
+		git checkout main &&
+		test_path_is_file Gitweb
 	)
 '