Use more efficient std::string::find overload.

This reflects an internal LSC cleanup.

Change-Id: Ic6d363ab14e0b021a579cdcf0a7a68a9021e2e18
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/44664
Commit-Queue: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/test/file_test.cc

diff --git a/crypto/test/file_test.cc b/crypto/test/file_test.cc
index c39c078..963429d 100644
--- a/crypto/test/file_test.cc
+++ b/crypto/test/file_test.cc
@@ -179,7 +179,7 @@
       kv = std::string(kv.begin() + 1, kv.end() - 1);
 
       for (;;) {
-        size_t idx = kv.find(",");
+        size_t idx = kv.find(',');
         if (idx == std::string::npos) {
           idx = kv.size();
         }