ExperimentalStateTracker: make suffix gracefully handle negative length

When requested to produce a suffix of a string of a string
of a given length, gracefully handle the case where the requested
length is negative---simply return the empty string in
this case. While there, mark this static method as such; also
increase visibility to default as it is generally useful and
should be tested as an interface of this class.

--
Change-Id: I821966f7ba3828809bc6d000358803c131740ec9
Reviewed-on: https://bazel-review.googlesource.com/#/c/4223
MOS_MIGRATED_REVID=129080284

src/main/java/com/google/devtools/build/lib/runtime/ExperimentalStateTracker.java

diff --git a/src/main/java/com/google/devtools/build/lib/runtime/ExperimentalStateTracker.java b/src/main/java/com/google/devtools/build/lib/runtime/ExperimentalStateTracker.java
index a3f4bce..2fa1197 100644
--- a/src/main/java/com/google/devtools/build/lib/runtime/ExperimentalStateTracker.java
+++ b/src/main/java/com/google/devtools/build/lib/runtime/ExperimentalStateTracker.java
@@ -33,7 +33,6 @@
 import com.google.devtools.build.lib.util.io.AnsiTerminalWriter;
 import com.google.devtools.build.lib.util.io.PositionAwareAnsiTerminalWriter;
 import com.google.devtools.build.lib.view.test.TestStatus.BlazeTestStatus;
-
 import java.io.IOException;
 import java.util.ArrayDeque;
 import java.util.Deque;
@@ -235,7 +234,10 @@
   /**
    * From a string, take a suffix of at most the given length.
    */
-  private String suffix(String s, int len) {
+  static String suffix(String s, int len) {
+    if (len <= 0) {
+      return "";
+    }
     int startPos = s.length() - len;
     if (startPos <= 0) {
       return s;