GNU testsuite comparison:

Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)

do i have to write the test cases also for the above fix ?

Yes, please :)

#[test]
fn test_multiple_files() {
    let (at, mut ucmd) = at_and_ucmd!();

    at.touch("file");
    at.touch("file1");

    ucmd.args(&["file1", "file1"])
        .succeeds()
        .stdout_contains("contents");
}

how to add content to the files
like

echo "contents" > file

You can use at.write("file", "content") instead of at.touch().

why this test is failing Style/spelling (ubuntu-latest, feat_os_unix)

It fails because "contenta" isn't a valid English word. I think you are missing a \n after "content" in your test_multiple_files() function:

.stdout_is("content\na        b");

running 1 test
write(default): /tmp/.tmpbmFGWl/file
write(default): /tmp/.tmpbmFGWl/file1
run: /home/biplab/project/coreutils/target/debug/coreutils unexpand file file1
thread 'test_unexpand::test_multiple_files' panicked at tests/by-util/test_unexpand.rs:259:10:
assertion failed: `(left == right)`

Diff < left / right > :
<contenta        b
>content
>a        b

the test is failing if \n is added the stdout is printing "contenta b" this only

Yes, you also have to adapt your implementation ;-)

Best to look at what GNU unexpand does:

$ echo "content" > file
$ echo "a   b" > file2
$ unexpand file file2
content
a   b

Here you can see, that GNU unexpand outputs "a b" on a new line.

Hope this helps.

biplab@BIPLAB:~/project/coreutils/src/uu/unexpand/src$ cargo run -- file file1
    Finished dev [unoptimized + debuginfo] target(s) in 0.27s
     Running `/home/biplab/project/coreutils/target/debug/unexpand file file1`
content
a        b
biplab@BIPLAB:~/project/coreutils/src/uu/unexpand/src$ unexpand file file1
content
a        b
biplab@BIPLAB:~/project/coreutils/src/uu/unexpand/src$ 

here the output is same to GNU i mean in the above it is printing on new line

in the test cases i don't why it is not adding \n for a new line

You are right, the implementation works fine :) I don't know yet why there is no \n in the test...

what should be next step
should i add extra space before "a b"
like " a b"
Style/spelling (ubuntu-latest, feat_os_unix)

this test case will not fail after that
or
it is alright

Ok, I figured out I was wrong and your test is correct. Sorry about that.

To make the Style/Spelling test pass, you can add

// spell-checker:ignore contenta

below the license info in test_unexpand.rs.

Thanks you for the suggestion

is my PR ready for merge ?

And merged, thanks for your PR :)

Thank you for your cooperation
i learned a lot again

Great to hear, you are welcome :)