wow64 mixed mode tests don't inject and child is native

I was trying to test >4GB drmarker reading for WOW64 AppInit on win8 (#1035), which
targets mixedmode, but our current tests don't seem to be injecting:

```
# bin64/drrun -debug -early_inject_map -early_inject_location 5 -heap_in_lower_4GB -- bin64/create_process.exe D:/derek/dr/build_suite/build_debug-internal-32/suite/tests/bin/win32.mixedmode.exe mixedmode
```

=> child is native

My original impl of earliest injection did not support a wow64 child
process, but I thought that Piyus had added it (xref #1507)?
Why do we have these tests then?

-early_inject_location 7 => still native

-early_inject => DR does inject but then crashes, b/c of an ARM bug (filed
as #1634): once fixed it works

This case covers figuring out the history here, and either disabling these
or implementing #1507 or whatever piece is missing.