Four tests in lib/std/posix/test.zig were disabled because they created
fixed-name files in the current working directory, and this caused
problems if tests were running in parallel with other build's tests.

This PR fixes those tests to all use `std.testing.tmpDir` to create unique
temporary names and directories.

Also clean the tests up to more consistently use `defer` to clean up, or
to just rely on tmpDir cleanup to remove individual files.

Working on these tests revealed a bunch of stale WASI code paths in
posix.zig, fixed by replacing stale `wast.AT.FDCWD` references with just
`AT.FDCWD`.

Fixes #14968.