test: make "missing path" assertions hermetic

[ronak-create]

Problem

Fifteen tool test suites hardcode an absolute path to assert not-found behaviour:

result, err := runWC([]string{"/nonexistent/file.txt"}, ...)

This assumes the path is absent from whatever machine runs the tests. That holds on no platform by guarantee, and on Windows it is actively wrong: /nonexistent/file.txt resolves to C:\nonexistent\file.txt, an ordinary creatable location.

When the path does exist, the assertion silently inverts — the test stops exercising the missing-file branch and starts exercising a real file.

This is not hypothetical. On a machine carrying a stray C:\nonexistent\path (a bare git repo, unrelated to this project), TestDu_NonExistent fails on master:

--- FAIL: TestDu_NonExistent (0.00s)
    du_test.go:173: expected error for non-existent path
FAIL    github.com/synseqack/aict/tools/du

du is correct here — it walked a directory that genuinely exists and correctly reported no errors. The test is what's broken. The other fourteen sites carry the identical latent fault and fail the same way given the right filesystem state.

Fix

Add internal/testutil.MissingPath, which anchors the path to t.TempDir() — a directory the test framework creates empty and removes on cleanup, so the leaf name is reliably absent on every platform:

func MissingPath(t *testing.T, name string) string {
	t.Helper()
	return filepath.Join(t.TempDir(), name)
}

Then swap all fifteen call sites (awk, cat, checksums, diff, du, file, find, head, ls, realpath, sed, stat, tail, tar, wc). Leaf names are preserved so extension-sensitive tools (file, tar) keep testing what they tested before.

Verification

• go test ./... — full suite green, with the stray C:\nonexistent\path still present on disk, which is what demonstrates the hermeticity rather than a cleanup.
• go vet ./... clean.
• gofmt -l reports no new entries (pre-existing formatting drift in non-test sources is untouched and out of scope here).

Test-only change: no tool behaviour is modified.