Consolidate and add new tests of adjacently tagged enums

[Mingun]

Similar to #2788, this PR moves all tests which tests top-level adjacently tagged enums (those which is defined using #[serde(tag = "t", content = "c")] attribute) into a dedicated file, removes duplicated tests and organizes tests in a logical manner. Flatten tests when adjacently tagged enum is flattened into structure I consider foremost as flatten tests, so they left untouched.

Removed tests which tests deserialization of generated tag and content field names from integers and bytes for each enum variant kind, such tests leaved only for Unit variant. This is because they are handled by the either TagOrContentFieldVisitor or TagContentOtherFieldVisitor, which can be tested only once.

Added tests includes deserialization from sequential representation, so this part of generated code becomes tested:

serde/serde_derive/src/de.rs

Lines 1680 to 1708 in 1a9ffdb

	fn visit_seq<__A>(self, mut __seq: __A) -> _serde::__private::Result<Self::Value, __A::Error>
	where
	__A: _serde::de::SeqAccess<#delife>,
	{
	// Visit the first element - the tag.
	match _serde::de::SeqAccess::next_element(&mut __seq)? {
	_serde::__private::Some(__field) => {
	// Visit the second element - the content.
	match _serde::de::SeqAccess::next_element_seed(
	&mut __seq,
	__Seed {
	field: __field,
	marker: _serde::__private::PhantomData,
	lifetime: _serde::__private::PhantomData,
	},
	)? {
	_serde::__private::Some(__ret) => _serde::__private::Ok(__ret),
	// There is no second element.
	_serde::__private::None => {
	_serde::__private::Err(_serde::de::Error::invalid_length(1, &self))
	}
	}
	}
	// There is no first element.
	_serde::__private::None => {
	_serde::__private::Err(_serde::de::Error::invalid_length(0, &self))
	}
	}
	}

[dtolnay]

Thank you!

These test suite PRs are much appreciated.