assertSnapshot tests fail if repo path is less than 3 levels deep

If the repo is checked out at a path less than 3 levels deep, tests that depend on CWD being at least 3 levels from / will fail. This poses problems in sandbox environments, where the code may be checked out at `/deno/std` instead of the typical `/home/somedenodev/std`. 1. Check out the repo in a `/code` directory under root 2. `cd /code/std` 3. `deno task ok` **Expected behavior** No `deno test ok` failures, per [CONTRIBUTING.md](https://github.com/denoland/std/blob/main/.github/CONTRIBUTING.md#pull-requests) and [AGENTS.md](https://github.com/denoland/std/blob/main/AGENTS.md#quality-gate): **Actual output** ``` assertSnapshot() - options ... mode => ./testing/snapshot_test.ts:287:11 error: AssertionError: unexpected output to stderr throw new AssertionError(msg); ^ at assert (file:///code/deno-std-pure/assert/assert.ts:22:11) at file:///code/deno-std-pure/testing/snapshot_test.ts:333:7 at async file:///code/deno-std-pure/testing/snapshot_test.ts:40:7 at async innerWrapped (ext:cli/40_test.js:230:5) at async exitSanitizer (ext:cli/40_test.js:146:27) at async Object.outerWrapped [as fn] (ext:cli/40_test.js:173:14) at async TestContext.step (ext:cli/40_test.js:564:22) at async file:///code/deno-std-pure/testing/snapshot_test.ts:287:3 assertSnapshot() - update => ./testing/snapshot_test.ts:338:6 error: AssertionError: Unexpected Error: Check ../../tmp/f8fdf6eb198f0022/test.ts throw new AssertionError(`Unexpected Error:\n\n${error}\n`); ^ at assertNoError (file:///code/deno-std-pure/testing/snapshot_test.ts:373:15) at file:///code/deno-std-pure/testing/snapshot_test.ts:392:5 at async file:///code/deno-std-pure/testing/snapshot_test.ts:40:7 assertSnapshot() - remove => ./testing/snapshot_test.ts:495:6 error: AssertionError: Unexpected Error: Check ../../tmp/c75f90cba5f40891/test.ts throw new AssertionError(`Unexpected Error:\n\n${error}\n`); ^ at assertNoError (file:///code/deno-std-pure/testing/snapshot_test.ts:524:15) at file:///code/deno-std-pure/testing/snapshot_test.ts:557:5 at async file:///code/deno-std-pure/testing/snapshot_test.ts:40:7 assertSnapshot() - different directory => ./testing/snapshot_test.ts:612:6 error: AssertionError: Unexpected Error: Check ../../tmp/ae46c315b1dc9240/test.ts Check ../../tmp/6e893da85387d0f6/test.ts throw new AssertionError(`Unexpected Error:\n\n${error}\n`); ^ at assertNoError (file:///code/deno-std-pure/testing/snapshot_test.ts:644:15) at file:///code/deno-std-pure/testing/snapshot_test.ts:688:5 at async file:///code/deno-std-pure/testing/snapshot_test.ts:58:7 FAILURES assertSnapshot() - options ... mode => ./testing/snapshot_test.ts:287:11 assertSnapshot() - update => ./testing/snapshot_test.ts:338:6 assertSnapshot() - remove => ./testing/snapshot_test.ts:495:6 assertSnapshot() - different directory => ./testing/snapshot_test.ts:612:6 ``` **Environment** - OS: Ubuntu 24.04 - deno version: 2.7.11 - std version: 1.0.18