`make test` leaves files behind

[juhlig]

Describe the bug
make test leaves behind a file lib/common_test/test_server/variables..

This was probably introduced with 63dc181

To Reproduce
For example:

$ cd $ERL_TOP/lib/stdlib
$ make ARGS="-suite queue_SUITE" test

After this, the new file is there.

Expected behavior
The file is removed again after the test has finished.

Affected versions
Noticed when using OTP 27.1

Additional context
I wouldn't mind the file too much, but sooner or later it is bound to end up in a PR if I don't pay attention.

[garazdawi]

The .gitignore in lib/common_test/test_server should be updated to variables* instead of variables, will you send a PR?