Also set TMPDIR in tests

[keith]

This is set to the same value as TEST_TMPDIR. I think the primary value for this in tests is that you might run tools that you do not control that have no knowledge of bazel, but likely respect this standard variable

Work towards https://github.com/bazelbuild/bazel/issues/27162

[jwnimmer-tri]

BTW any changes to the test environment should probably simultaneously update the documentation: https://bazel.build/reference/test-encyclopedia#initial-conditions