Add environment variable to activate snapshot writing

snapshot-matcher/src/main/java/com/zenika/snapshotmatcher/SnapshotMatcher.java

@@ -11,6 +11,7 @@
 import java.nio.file.Paths;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Optional;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
@@ -48,11 +49,14 @@ public boolean matchesSafely(T o) {
         if (Files.exists(snapshotPath)) {
             // File exists => Compare snapshot file to given object
             return compareSnapshot(o, snapshotPath);
-        } else {
+        } else if (isWriteSnapshotActivated()) {
             // File doesn't exist => Create snapshot file and return true
             createSnapshot(o, snapshotPath);
             return true;
         }
+        System.out.println("Snapshot writing is not activated in this environment.");
+        System.out.println("Activate snapshot writing by using -Dtest.snapshots.write");
+        return false;
     }
 
     /**
@@ -153,4 +157,7 @@ private StackTraceElement getCaller() {
                 .orElse(null);
     }
 
+    private boolean isWriteSnapshotActivated() {
+        return System.getProperty("test.snapshots.write") != null;
+    }
 }