Revert "Use relative paths for filenames in deprecation warnings"

This reverts commit 6bd1ec9.

Author: Vasileios Karakasis

reframe/core/warnings.py

@@ -1,6 +1,5 @@
 import contextlib
 import inspect
-import os
 import warnings
 
 from reframe.core.exceptions import ReframeFatalError
@@ -31,8 +30,6 @@ def _format_warning(message, category, filename, lineno, line=None):
             except IndexError:
                 line = '<no line information>'
 
-    # Use a relative path
-    filename = os.path.relpath(filename)
     message = f'{filename}:{lineno}: WARNING: {message}\n{line}\n'
 
     # Ignore coloring if runtime has not been initialized; this can happen