Lower the compile threshold and reduce the iterations of warmup loop in VarHandles tests

java/lang/invoke/VarHandles tests run with C1, C2 and tiered compilations and run the test cases in the warm up loop with 20000 iterations to verify C1, C2 intrinsics. Default Tier4CompileThreshold is 15000.

Scale the compile threshold to 0.1 will allow the warm up loop to reduce the number of iterations and the test execution time.

$ java -XX:CompileThresholdScaling=0.1 -XX:+PrintFlagsFinal | grep CompileThreshold

     intx CompileThreshold = 1000 {pd product} {ergonomic}
   double CompileThresholdScaling = 0.100000 {product} {command line}
    uintx IncreaseFirstTierCompileThresholdAt = 50 {product} {default}
     intx Tier2CompileThreshold = 0 {product} {default}
     intx Tier3CompileThreshold = 200 {product} {ergonomic}
     intx Tier4CompileThreshold = 1500 {product} {ergonomic}