Run compiletest on linux-dev

The compiletest test is quick to run and shouldn't increase
build times very much.

buildbot/master/files/config/master.cfg

@@ -105,6 +105,7 @@ mac_test_env = dict({
 linux_dev_factory = create_servo_factory([
     steps.ShellCommand(command=["./mach", "test-tidy", "--no-progress"], env=linux_headless_env),
     steps.Compile(command=["./mach", "build", "--dev"], env=linux_headless_env),
+    steps.ShellCommand(command=["./mach", "test-compiletest"], env=linux_headless_env),
     steps.ShellCommand(command=["./mach", "test-unit"], env=linux_headless_env),
     steps.Compile(command=["./mach", "build-cef"], env=linux_headless_env),
     steps.Compile(command=["./mach", "build-geckolib"], env=linux_headless_env),