According to this example extracted from tap documentation, the
skipped and pending tests should be included in the plan:
```
TAP version 14
1..2

ok 1 - do it later # Skipped

ok 2 - works on windows # Skipped: only run on windows
```

Give the variable an explicit name.
Use mktemp for better portability.
Use trap to ensure we remove this file at the end, whatever happens.