t6025: use helpers to replace test -f <path>

Take advantage of helper function 'test_path_is_file()' to
replace 'test -f' since the function makes the code more
readable and gives better error messages.

Signed-off-by: Shourya Shukla <shouryashukla.oo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6025-merge-symlinks.sh

@@ -36,7 +36,7 @@ test_expect_success 'merge master into b-symlink, which has a different symbolic
 '
 
 test_expect_success 'the merge result must be a file' '
-	test -f symlink
+	test_path_is_file symlink
 '
 
 test_expect_success 'merge master into b-file, which has a file instead of a symbolic link' '
@@ -46,7 +46,7 @@ test_expect_success 'merge master into b-file, which has a file instead of a sym
 '
 
 test_expect_success 'the merge result must be a file' '
-	test -f symlink
+	test_path_is_file symlink
 '
 
 test_expect_success 'merge b-file, which has a file instead of a symbolic link, into master' '
@@ -56,7 +56,7 @@ test_expect_success 'merge b-file, which has a file instead of a symbolic link,
 '
 
 test_expect_success 'the merge result must be a file' '
-	test -f symlink
+	test_path_is_file symlink
 '
 
 test_done