Added new flag 'run_test' to support test steps for test exercises.

Also created a simple exercise '102_testing' to test the new flag.
After the new build system is ready, we skip the exercise in the flow to finish the actual testing exercise.

exercises/102_testing.zig

@@ -0,0 +1,10 @@
+const std = @import("std");
+const testing = std.testing;
+
+fn add(a: u16, b: u16) u16 {
+    return a + b;
+}
+
+test "simple test" {
+    try testing.expect(add(41, 1) == 42);
+}