🧪 Add test for synonym types (#128)

The `analyzer` package currently has tests for type aliases that define
a new type, like this:

```go
type TestAlias Test
```

But not for type aliases that _don't_ define a new type but only a new
symbol, like this:

```go
type TestSynonym = Test
```

I've called this second category "synonyms" but Go's documentation calls
them "aliases". What the current tests call "aliases" is generally seen
as a distinct type with the same memory layout.

This PR adds tests for these "synonyms". All of these tests pass. The PR
can be merged with no impact on any of the analyzer's behaviour.

**Why am I adding these tests?**

Because while these tests pass with Go 1.21, they fail with Go 1.23 and
above. I haven't figured out what change in Go 1.23 breaks the analyzer
for these cases yet. But when it comes time to fix the analyzer, these
tests will be useful.

**The linter is buggy in `golangci-lint` now.**

When compiling `go-exhaustruct` directly, the `go 1.21` directive in
`go.mod` makes sure the behaviour works as expected. However,
`golangci-lint` does not compile `go-exhaustruct` directly: it imports
the analyzer and then compiles `golangci-lint` as a whole. A few days
ago, `golangci-lint` changed their `go 1.22` directive to `go 1.23`,
which silently broke the `exhaustruct` linter.

Starting with `golangci-lint` v1.65.0, the `exhaustruct` linter does not
detect missing fields for types with an alias. Here is the smallest
example I could build to reproduce:

```go
package foo

type Foo struct {
	A int
	B int
}

type FooAlias = Foo

func MyFunc() {
	_ = FooAlias{
		A: 1,
	}
}
```

When running linters on the code above, we observe this behaviour:

✅ `golangci-lint` v1.64.x detects the missing field.
❌ `golangci-lint` v1.65.x does not detect the missing field.
✅ `go-exhaustruct` v3.3.0 compiled with Go 1.22 detects the missing field.
❌ `go-exhaustruct` v3.3.0 compiled with Go 1.23 does not detect the missing field.

I plan to open an issue on `golangci-lint` as well, because it's their
upgrade that broke the linter. However I think the fix will be easiest
to implement in `go-exhaustruct`.

analyzer/testdata/src/i/i.go

@@ -223,3 +223,28 @@ func shouldFailExternalTypeAliases() {
 	_ = TestExternalAliasAlias{}    // want "i.TestExternalAliasAlias is missing fields A, B"
 	_ = TestExternalExcludedAlias{} // want "i.TestExternalExcludedAlias is missing fields A, B"
 }
+
+type TestSynonym = Test
+type TestSynonymSynonym = TestSynonym
+type TestSynonymAlias TestSynonym
+type TestSynonymExcluded = Test
+
+func shouldFailTypeSynonyms() {
+	_ = TestSynonym{}         // want "i.Test is missing fields A, B, C, D"
+	_ = TestSynonymSynonym{}  // want "i.Test is missing fields A, B, C, D"
+	_ = TestSynonymAlias{}    // want "i.TestSynonymAlias is missing fields A, B, C, D"
+	_ = TestSynonymExcluded{} // want "i.Test is missing fields A, B, C, D"
+}
+
+type TestExternalSynonym = e.External
+type TestExternalSynonymSynonym = TestExternalSynonym
+type TestExternalExcludedSynonym = e.ExternalExcluded
+
+func shouldFailExternalTypeSynonyms() {
+	_ = TestExternalSynonym{}        // want "e.External is missing fields A, B"
+	_ = TestExternalSynonymSynonym{} // want "e.External is missing fields A, B"
+}
+
+func shouldSucceedExcludedSynonyms() {
+	_ = TestExternalExcludedSynonym{}
+}