Temp helpers allow paths outside test directory, breaking isolation

### Summary

Temp file and directory helpers in `absl.testing.absltest` currently accept relative paths that can escape the test’s temporary directory (e.g., via `..`). This can create files or directories outside the intended test sandbox.

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### Reproducible example

```python
from absl.testing import absltest

class ExampleTest(absltest.TestCase):
def test_escape(self):
tf = self.create_tempfile('../outside.txt')
print(tf.full_path) # points outside the test temp directory
```

Running this creates a file outside the test-specific temp directory.

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### Observed behavior

* Relative paths like `"../outside.txt"` are accepted
* Resulting paths may resolve outside the test temp directory
* Cleanup may affect unintended directories depending on path structure

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### Expected behavior

All temp helpers should ensure that created files and directories remain within the test’s temporary directory to preserve isolation guarantees.

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### Why this matters

* Breaks test isolation (tests can write outside their sandbox)
* Makes cleanup behavior less predictable
* Can lead to interference between tests or with local filesystem state

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### Proposed direction

Reject paths that:

* contain traversal components (e.g., `..`)
* are absolute paths

This would ensure all helper-created paths remain confined to the test temp directory.

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### Notes

* This issue focuses on maintaining expected test isolation behavior
* No changes are proposed to valid relative paths within the temp directory