reftable: wrap EXPECT macros in do/while

reftable: wrap EXPECT macros in do/while

The `EXPECT` macros used by the reftable test framework are all using a
single `if` statement with the actual condition. This results in weird
syntax when using them in if/else statements like the following:

```
if (foo)
	EXPECT(foo == 2)
else
	EXPECT(bar == 2)
```

Note that there need not be a trailing semicolon. Furthermore, it is not
immediately obvious whether the else now belongs to the `if (foo)` or
whether it belongs to the expanded `if (foo == 2)` from the macro.

Fix this by wrapping the macros in a do/while loop.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>