CTest: Delete tmpdir if it wasn't previously

If a previous test run was interrupted, it might not have cleaned up any temp directories and a subsequent run could fail on `mkdir`. Delete the temporary directory before trying to create it.

Depends on https://github.com/google/iree/pull/775

Closes https://github.com/google/iree/pull/778

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/778 from GMNGeoffrey:ctest-cleanup 2339d9aa2e1f5a0bb3aa43b4f2b6e12c6fd7057b
PiperOrigin-RevId: 295770807
diff --git a/build_tools/cmake/run_test.sh b/build_tools/cmake/run_test.sh
index 50eed16..0248fc1 100755
--- a/build_tools/cmake/run_test.sh
+++ b/build_tools/cmake/run_test.sh
@@ -26,6 +26,7 @@
 }
 
 echo "Creating test environment"
+rm -rf "${TEST_TMPDIR?}" # In case this wasn't cleaned up previously
 mkdir "${TEST_TMPDIR?}"
 trap cleanup EXIT
 # Execute whatever we were passed.