Since Android Gradle plugin version 1.3.0 you can use
./gradlew -Pandroid.testInstrumentationRunnerArguments.class=your.package.LandingActivityTests connectedAndroidTest
Since Android Gradle plugin version 1.3.0 you can use
./gradlew -Pandroid.testInstrumentationRunnerArguments.class=your.package.LandingActivityTests connectedAndroidTest