Do not initialize / shutdown JIT for each test

This commit avoid initialization and shutdown of JIT for *each* TRIL test
and instead shutdown / initialize JIT again every 5000 tests (this can
be easily changed).

to further reduce memory footprint of `comptest`. As in case of type
dictionary, during JIT initialization some internal structures are
allocated in persistent memory and thus never freed. More importantly,
this commit helps significantly to reduce memory footprint when running
tests under QEMU - the exact nature of why this help escapes me, but
probably has to do with memory used by QEMU's TCG "JIT".

With this change, running tests for x86_64 natively, the peak memory
consumption of `comptest` drops by another ~150MB (to 317MB). For RISC-V
using QEMU on x86_64 host peak memory consumption drops to 460MB.

fvtest/compilertest/control/TestJit.cpp

@@ -178,30 +178,66 @@ initializeTestJit(TR_RuntimeHelper *helperIDs, void **helperAddresses, int32_t n
    return true;
    }
 
+extern "C"
+void
+shutdownTestJit()
+   {
+   auto fe = TestCompiler::FrontEnd::instance();
+
+   TR::CodeCacheManager &codeCacheManager = fe->codeCacheManager();
+   codeCacheManager.destroy();
+
+   TR::CompilationController::shutdown();
+   }
+
+
+static int  initCount = 0;
+const  int  initCountMax = 5000;
+static std::string initOptions = "";
+
 extern "C"
 bool
 initializeJitWithOptions(char *options)
    {
-   return initializeTestJit(0, 0, 0, options);
+   if (// If initialized with different options...
+          (initCount > 0 && initOptions != options)
+       // ...or virtual "initialication" count reaches threshold
+          || (initCount > initCountMax))
+      {
+      shutdownTestJit();
+      initCount = 0;
+      initOptions = "";
+      }
+
+   // Initialize if not initialized already
+   if (initCount == 0)
+      {
+      if (initializeTestJit(0, 0, 0, options))
+         {
+         initCount = 1;
+         initOptions = options;
+         }
+      }
+   // else increment "initialization" count.
+   else
+      {
+         initCount++;
+      }
+   return initCount > 0;
    }
 
 extern "C"
 bool
 initializeJit()
    {
-   return initializeTestJit(0, 0, 0, (char *)"-Xjit:useILValidator");
+   return initializeJitWithOptions((char *)"-Xjit:useILValidator");
    }
 
 extern "C"
 void
 shutdownJit()
    {
-   auto fe = TestCompiler::FrontEnd::instance();
-
-   TR::CodeCacheManager &codeCacheManager = fe->codeCacheManager();
-   codeCacheManager.destroy();
-
-   TR::CompilationController::shutdown();
+   // Do nothing, shutdown is handled in initializeJitWithOptions()
    }
 
 extern "C"