fix(test): update expected format for unrecognised extension to yaml (#2837)

When unrecognised extensions are passed, FormatStringFromFilename defaults to 'yaml' (as handled in pkg/yqlib/format.go and tested in pkg/yqlib/format_test.go).

This updates the test expectation in test/format_test.go for 'file.unknown' to 'yaml' so that go test ./test passes.

test/format_test.go

@@ -17,7 +17,7 @@ func TestFormatStringFromFilename(t *testing.T) {
 		{"FILE.JSON", "json"},
 		{"file.properties", "properties"},
 		{"file.xml", "xml"},
-		{"file.unknown", "unknown"},
+		{"file.unknown", "yaml"},
 
 		// filenames without extensions
 		{"file", "yaml"},