Add embedder tests for public/fpdf_searchex.h.

Exercise FPDFText_GetCharIndexFromTextIndex() and
FPDFText_GetTextIndexFromCharIndex().

Change-Id: I0e74b0cd88cbc7bf28d046dd24829c0dde0a39f1
Reviewed-on: https://pdfium-review.googlesource.com/c/50692
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/fpdfsdk/BUILD.gn b/fpdfsdk/BUILD.gn
index adea01c..68de270 100644
--- a/fpdfsdk/BUILD.gn
+++ b/fpdfsdk/BUILD.gn
@@ -124,6 +124,7 @@
     "fpdf_formfill_embeddertest.cpp",
     "fpdf_ppo_embeddertest.cpp",
     "fpdf_save_embeddertest.cpp",
+    "fpdf_searchex_embeddertest.cpp",
     "fpdf_structtree_embeddertest.cpp",
     "fpdf_sysfontinfo_embeddertest.cpp",
     "fpdf_text_embeddertest.cpp",