[api-minor] Ensure that the AnnotationLayer gets a default l10n-instance in GENERIC builds (PR 16552 follow-up)

*This is something that I completely overlooked during review of PR 16552, despite leaving a l10n-related comment.*

The new l10n-handling of PopupAnnotations assume that the `AnnotationLayer` is always initialized with a l10n-instance, which might not actually be the case in third-party implementations where the default viewer isn't used.
To work-around that we'll now bundle, and fallback on, the existing `NullL10n`-implementation in GENERIC builds of the PDF.js library. This will only result in a slight file-size increase for the *built* `pdf.js` file, again limited to GENERIC builds, since the `web/l10n_utils.js` file has no dependencies.

Also, tweaks a couple of TESTING pre-processor checks to *only* include that code when running the reference tests.

web/l10n_utils.js

@@ -13,6 +13,13 @@
  * limitations under the License.
  */
 
+/**
+ * PLEASE NOTE: This file is currently imported in both the `web/` and
+ *              `src/display/` folders, hence be EXTREMELY careful about
+ *              introducing any dependencies here since that can lead to an
+ *              unexpected/unnecessary size increase of the *built* files.
+ */
+
 /**
  * A subset of the l10n strings in the `l10n/en-US/viewer.properties` file.
  */
@@ -63,6 +70,8 @@ const DEFAULT_L10N_STRINGS = {
   unexpected_response_error: "Unexpected server response.",
   rendering_error: "An error occurred while rendering the page.",
 
+  annotation_date_string: "{{date}}, {{time}}",
+
   printing_not_supported:
     "Warning: Printing is not fully supported by this browser.",
   printing_not_ready: "Warning: The PDF is not fully loaded for printing.",