[sed, test] tests should not drop temporary files in the source code directory

[jgarzik]

Running tests results in unclean git repository, which is an error:

% git status
On branch main
Your branch is up to date with 'origin/main'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
        modified:   text/tests/sed/assets/newfile

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        text/atyfv

no changes added to commit (use "git add" and/or "git commit -a")

Any test data modification should occur in target/tmp.

cc @Wandalen

[jgarzik]

Hint: Please use env!("CARGO_TARGET_TMPDIR") for the tempdir location base directory.

[jgarzik]

fixed by #406