compiler: add regression test for generic methods in interfaces

Add compiler/testdata/go1.27.go, gated behind Go >= 1.27 (the version
that promoted generic methods out of the GenericMethods experiment),
covering both fixes from the previous two commits:

  - genericMethod has a regular method and a generic method (its own
    type parameter); boxing it into an interface must only include the
    regular method in the runtime method set instead of panicking in
    getTypeCodeName.
  - onlyGenericMethod's sole method is generic, so its type code must
    have hasMethodSet == false and no methodSet field at all, not an
    empty one.

Verified this test panics on the pre-fix compiler/interface.go and
passes after it.

compiler/compiler_test.go

@@ -60,6 +60,9 @@ func TestCompiler(t *testing.T) {
 	if goMinor >= 21 {
 		tests = append(tests, testCase{"go1.21.go", "", ""})
 	}
+	if goMinor >= 27 {
+		tests = append(tests, testCase{"go1.27.go", "", ""})
+	}
 
 	for _, tc := range tests {
 		name := tc.file