test: Update FIT tests to run in parallel

Use a different temporary dir for each test, to allow them to run in
parallel.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/py/tests/test_fit_ecdsa.py b/test/py/tests/test_fit_ecdsa.py
index 87b6081..cc6c0c4 100644
--- a/test/py/tests/test_fit_ecdsa.py
+++ b/test/py/tests/test_fit_ecdsa.py
@@ -10,6 +10,7 @@
 This test doesn't run the sandbox. It only checks the host tool 'mkimage'
 """
 
+import os
 import pytest
 import u_boot_utils as util
 from Cryptodome.Hash import SHA256
@@ -84,7 +85,8 @@
     cons = u_boot_console
     mkimage = cons.config.build_dir + '/tools/mkimage'
     datadir = cons.config.source_dir + '/test/py/tests/vboot/'
-    tempdir = cons.config.result_dir
+    tempdir = os.path.join(cons.config.result_dir, 'ecdsa')
+    os.makedirs(tempdir, exist_ok=True)
     key_file = f'{tempdir}/ecdsa-test-key.pem'
     fit_file = f'{tempdir}/test.fit'
     dtc('sandbox-kernel.dts')