integrationtests: scope leaked-temp-dir detection to a per-test prefix (q10)

TestCleanupLeakedWorkloadTempDirCaughtByAssertion was order/concurrency
dependent: passed in isolation but its detection scanned the shared system
temp root (os.TempDir) for ANY "ioworkload-" directory via a before/after
diff. Real ioworkload scenario workloads legitimately create
ioworkload-<scenario>-* dirs in that same root while they run (in parallel
under `mage integrationTest`), so this test both falsely attributed them as
leaks and, worse, RemoveAll'd them out from under the still-running tests.

Fix: scope both detection and cleanup to a prefix unique to this single test
invocation (PID + sanitized t.Name()) via listWorkloadTempDirsWithPrefix and
newLeakedDirs. Other tests' temp dirs can no longer be observed or deleted.

Also fixed a latent detection bug: the intentional-leak workload script
hard-coded /tmp instead of os.TempDir(), so detection silently failed whenever
$TMPDIR differed from /tmp. The mktemp template now uses os.TempDir().

Removed the dead assertNoNewWorkloadTempDirs helper (defined, never called).

Verified: mage build OK; mage test green across repeated runs; mage generate
produces no diff; gofmt clean. A stress run with decoy ioworkload-<scenario>-*
dirs confirms they survive untouched.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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