all: rename "test_files" dirs to Go idiomatic "testdata".

The Go "testdata" convention (as respected by the cmd/go tool)
is briefly documented in https://pkg.go.dev/cmd/go

labeler/hiveai_test.go

···
 )
 
 func TestHiveParse(t *testing.T) {
-	file, err := os.Open("test_files/hiveai_resp_example.json")
+	file, err := os.Open("testdata/hiveai_resp_example.json")
 	if err != nil {
 		t.Fatal(err)
 	}

repomgr/ingest_test.go

···
 
 	repoman := NewRepoManager(maindb, cs, &util.FakeKeyManager{})
 
-	fi, err := os.Open("../testing/test_files/divy.repo")
+	fi, err := os.Open("../testing/testdata/divy.repo")
 	if err != nil {
 		t.Fatal(err)
 	}

testing/feedpost_test.go

···
 	assert := assert.New(t)
 
 	// this is a post-lex-refactor app.bsky.feed.post record
-	inFile, err := os.Open("test_files/feedpost_record.cbor")
+	inFile, err := os.Open("testdata/feedpost_record.cbor")
 	assert.NoError(err)
 	cborBytes, err := io.ReadAll(inFile)
 	assert.NoError(err)

testing/repo_slice_test.go

···
 
 func TestRepoSliceParse(t *testing.T) {
 	ctx := context.TODO()
-	fi, err := os.Open("test_files/repo_slice.car")
+	fi, err := os.Open("testdata/repo_slice.car")
 	if err != nil {
 		t.Fatal(err)
 	}

testing/sig_test.go

···
 func TestVerification(t *testing.T) {
 	assert := assert.New(t)
 
-	fi, err := os.Open("test_files/divy.repo")
+	fi, err := os.Open("testdata/divy.repo")
 	if err != nil {
 		t.Fatal(err)
 	}