Adjust test to work in more environments. (#1231)

The current test is really testing too closely to bazel internals.

explorer/fuzzing/fuzzer_util_test.cpp

@@ -39,14 +39,9 @@ TEST(FuzzerUtilTest, RunFuzzerOnCorpus) {
 TEST(FuzzerUtilTest, GetRunfilesFile) {
   EXPECT_THAT(*Internal::GetRunfilesFile(
                   "carbon/explorer/fuzzing/fuzzer_corpus/empty.textproto"),
-              testing::EndsWith("/explorer/fuzzing/fuzzer_corpus/"
-                                "empty.textproto"));
-  EXPECT_THAT(Internal::GetRunfilesFile(
-                  "explorer/fuzzing/fuzzer_corpus/empty.textproto")
-                  .error()
-                  .message(),
-              testing::EndsWith("fuzzer_util_test.runfiles/explorer/fuzzing/"
-                                "fuzzer_corpus/empty.textproto doesn't exist"));
+              testing::EndsWith("/empty.textproto"));
+  EXPECT_THAT(Internal::GetRunfilesFile("nonexistent-file").error().message(),
+              testing::EndsWith("doesn't exist"));
 }
 
 }  // namespace