Added clean task to smoke tests. (#527)

This change allows the smoke tests to clean all build variants created
by the infrastructure.

Author: allisonbm92

smoke-tests/build.gradle

@@ -12,6 +12,10 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+import java.nio.file.Files
+import java.nio.file.Path
+import java.nio.file.Paths
+
 buildscript {
   repositories {
     google()
@@ -114,4 +118,12 @@ dependencies {
   storageImplementation "com.google.firebase:firebase-storage"
 }
 
+clean.doLast {
+  def paths = Files.newDirectoryStream(Paths.get("."), "build-*")
+
+  for (Path path : paths) {
+    project.delete "$path/"
+  }
+}
+
 apply plugin: "com.google.gms.google-services"