tools: fix TestFdtUtil()

Variable old_outdir cannot be used before assignment.
The assignment must occur before the try block.

tools/dtoc/test_fdt.py:796:26:
E0601: Using variable 'old_outdir' before assignment
(used-before-assignment)

Add missing space in assignment.

Fixes: a004f294 ("binman: Tidy up _SetupDtb() to use its own temporary file")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>