Allow `requires` tag with tests.

[ThHareau]

For now, it is possible to use requires inside comparators, tags, matchtype and addresspart.

The relational extension uses that for the count match type:

	<matchtype name="count">
		<requires type="comparator" name="i;ascii-numeric" regex="i;ascii-numeric" />
		<parameter type="string" name="relation string" regex="(lt|le|eq|ge|gt|ne)" />
	</matchtype>

It would be useful to be able to define a test that requires a specific field.