L2CacheTest: pass split lines to verilator backend,

this greatly accelerates compilation.

src/test/scala/cache/L2CacheTest.scala

@@ -5,6 +5,7 @@ import chisel3._
 import chisel3.util._
 import chiseltest.experimental.TestOptionBuilder._
 import chiseltest.internal.{VerilatorBackendAnnotation, LineCoverageAnnotation, ToggleCoverageAnnotation, UserCoverageAnnotation, StructuralCoverageAnnotation}
+import chiseltest.legacy.backends.verilator.VerilatorFlags
 import chiseltest._
 import chisel3.experimental.BundleLiterals._
 import firrtl.stage.RunFirrtlTransformAnnotation
@@ -262,6 +263,7 @@ class L2CacheTest extends AnyFlatSpec with ChiselScalatestTester with Matchers{
     ToggleCoverageAnnotation,
     UserCoverageAnnotation,
     StructuralCoverageAnnotation,
+    VerilatorFlags(Seq("--output-split 5000", "--output-split-cfuncs 5000")),
     RunFirrtlTransformAnnotation(new PrintModuleName)
   )