Create seedfile before running tests

With MBEDTLS_USE_PSA_CRYPTO and MBEDTLS_ENTROPY_NV_SEED enabled, the
tests need a seedfile. Since test_suite_entropy is no longer there to
create it, and MBEDTLS_USE_PSA_CRYPTO is now enabled in the full
config, create tests/seedfile explicitly in basic-build-test.sh.
diff --git a/tests/scripts/basic-build-test.sh b/tests/scripts/basic-build-test.sh
index ab95e22..2c9deb9 100755
--- a/tests/scripts/basic-build-test.sh
+++ b/tests/scripts/basic-build-test.sh
@@ -74,6 +74,9 @@
 # Step 2 - Execute the tests
 TEST_OUTPUT=out_${PPID}
 cd tests
+if [ ! -f "seedfile" ]; then
+    dd if=/dev/urandom of="seedfile" bs=32 count=1
+fi
 
 # Step 2a - Unit Tests
 perl scripts/run-test-suites.pl -v 2 |tee unit-test-$TEST_OUTPUT