refactor(tests): update test helpers and all test file references

- Rename createTestEbookID to createTestMediaItemID in test_helpers.go
- Update test helper comments and variable names (ebookReq -> mediaItemReq, etc.)
- Update all test file references:
  - analytics_test.go
  - book_matching_test.go
  - collections_bulk_test.go
  - kobo_test.go
  - media_bulk_test.go
  - opds_test.go
- Rename ebookID variable to mediaItemID in kobo_test.go
- Update test data to use 'Test Media Item' instead of 'Test Ebook'

cmd/server/tests/book_matching_test.go

@@ -38,7 +38,7 @@ func TestBookMatchingQueryBooks(t *testing.T) {
 		defer ts.Close()
 
 		token := loginTestUser(t, ts, db)
-		_ = createTestEbookID(t, ts, token)
+		_ = createTestMediaItemID(t, ts, token)
 
 		req := map[string]interface{}{
 			"title": "Test Ebook",
@@ -180,7 +180,7 @@ func TestBookMatchingBulkLink(t *testing.T) {
 		defer ts.Close()
 
 		token := loginTestUser(t, ts, db)
-		bookID := createTestEbookID(t, ts, token)
+		bookID := createTestMediaItemID(t, ts, token)
 
 		req := map[string]interface{}{
 			"links": []map[string]interface{}{