Error parsing if/else block in test-suite stanza of cabal file

Using an `if`/`else` block to provide alternative `main-is` values in the test-suite stanza results in a parsing error:

> Test suite "cabal-bug" is missing required field "type" or the field is not present in all conditional branches.

[Minimal reproducible example.](https://gist.github.com/nghamilton/0a2240a189f66432ea090027b27f7469)

A workaround is to include `type` field in both branches, as suggested by the error message:

```
if flag(breaks)
main-is: Main1.hs
type: exitcode-stdio-1.0
else
main-is: Main2.hs
type: exitcode-stdio-1.0
```

If this is the expected behavior, the error message could possibly be expressed more clearly, or the documentation could state the `type` field should be present in all conditional branches.

Tested on cabal version 3.0.0.