ART: Slow down dex2oat for watchdog test

Try to slow down dex2oat by using an excessive number of (idle)
threads.

Bug: 63052624
Test: m test-art-host-gtest-dex2oat_test
Change-Id: Ifda3f4407eea2ec2b025b1d576fe14840ac6be75
diff --git a/dex2oat/dex2oat_test.cc b/dex2oat/dex2oat_test.cc
index 75859ca..21768d3 100644
--- a/dex2oat/dex2oat_test.cc
+++ b/dex2oat/dex2oat_test.cc
@@ -968,6 +968,7 @@
 
     std::string swap_location = GetOdexDir() + "/Dex2OatSwapTest.odex.swap";
     copy.push_back("--swap-file=" + swap_location);
+    copy.push_back("-j512");  // Excessive idle threads just slow down dex2oat.
     GenerateOdexForTest(dex_location,
                         odex_location,
                         CompilerFilter::kSpeed,