Use mock network tables in tests (#675)

Author: SamCarlberg

core/src/test/java/edu/wpi/grip/core/operations/network/networktables/MockNTReceiver.java

@@ -0,0 +1,39 @@
+package edu.wpi.grip.core.operations.network.networktables;
+
+import edu.wpi.grip.core.operations.network.NetworkReceiver;
+
+import edu.wpi.first.wpilibj.tables.ITable;
+
+import java.util.function.Consumer;
+
+public class MockNTReceiver extends NetworkReceiver {
+
+  private final ITable table;
+
+  /**
+   * Create a new NetworkReceiver with the specified path.
+   *
+   * @param path  The path of the object to get
+   * @param table the network table that values will be retrieved from
+   */
+  public MockNTReceiver(String path, ITable table) {
+    super(path);
+    this.table = table;
+  }
+
+  @Override
+  public Object getValue() {
+    return table.getValue(path);
+  }
+
+  @Override
+  public void addListener(Consumer<Object> consumer) {
+    // Never called
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public void close() {
+    table.delete(path);
+  }
+}