test-theme: Check return value of chdir()

test-theme: Check return value of chdir()

I noticed the following warning while building the .deb package for
gnome-shell on Endless OS:

      ../src/st/test-theme.c: In function ‘main’:
      ../src/st/test-theme.c:549:3: warning: ignoring return value of ‘chdir’, declared with attribute warn_unused_result [-Wunused-result]
         chdir (cwd);
         ^~~~~~~~~~~

(Of course this is very unlikely to fail in practice.)

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1208