logcat: test: also check "failed to clear" in blocking_clear

Add more detail about logcat.blocking_clear test failure

Test: gTest logcat-unit-test --gtest_filter=logcat.blocking_clear
Bug: 37483775
Change-Id: I6555c13593781a1278d9106e66c2dec6009a2215
diff --git a/logcat/tests/logcat_test.cpp b/logcat/tests/logcat_test.cpp
index e487a97..9c07472 100644
--- a/logcat/tests/logcat_test.cpp
+++ b/logcat/tests/logcat_test.cpp
@@ -1216,7 +1216,12 @@
     signal(SIGALRM, caught_blocking_clear);
     alarm(2);
     while (fgets(buffer, sizeof(buffer), fp)) {
-        if (!strncmp(buffer, "clearLog: ", 10)) {
+        if (!strncmp(buffer, "clearLog: ", strlen("clearLog: "))) {
+            fprintf(stderr, "WARNING: Test lacks permission to run :-(\n");
+            count = signals = 1;
+            break;
+        }
+        if (!strncmp(buffer, "failed to clear", strlen("failed to clear"))) {
             fprintf(stderr, "WARNING: Test lacks permission to run :-(\n");
             count = signals = 1;
             break;