test: fix failure in `test/sequential/test-heapdump.js`

The test was failing when it was being run with superuser privileges, so this changes the test from attempting to write to a read-only file to attempting to write to a file with the same name as that of an existing directory, as that is a more reliable way of making v8.writeHeapSnapshot() throw even when run with sudo.

Fixes: https://github.com/nodejs/node/issues/41643 Signed-off-by: Darshan Sen raisinten@gmail.com