make 'cwd' param for TerminalManager absolute

[rccern]

needed for jupyterlab/jupyterlab#12250 as discussed with @fcollonval
I was also thinking about a test. If here https://github.com/jupyter-server/jupyter_server/blob/main/tests/test_terminal.py#L83 I put str(os.path.basename(terminal_path)) then the test 'hangs' (as expected, but it's not ideal in the test). Otherwise we can check for str(terminal_path) instead of os.path.basename here https://github.com/jupyter-server/jupyter_server/blob/main/tests/test_terminal.py#L136

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[codecov-commenter]

Codecov Report

Merging #749 (0964b23) into main (3864399) will decrease coverage by 0.02%.
The diff coverage is 33.33%.

❗ Current head 0964b23 differs from pull request most recent head b92362d. Consider uploading reports for the commit b92362d to get more accurate results

@@            Coverage Diff             @@
##             main     #749      +/-   ##
==========================================
- Coverage   70.51%   70.48%   -0.03%     
==========================================
  Files          62       62              
  Lines        7615     7621       +6     
  Branches     1216     1218       +2     
==========================================
+ Hits         5370     5372       +2     
- Misses       1877     1880       +3     
- Partials      368      369       +1     

Impacted Files	Coverage Δ
jupyter_server/terminal/api_handlers.py	89.47% <33.33%> (-10.53%)	⬇️

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[blink1073]

Thanks!

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