Cleanup based on Michal's PR feedback

I have modified the change along Michal's suggestions. I have
removed the explicit setting of environment variables in the
tests\runtest.cmd script and instead I just added logic for forwarding
the gcstresslevel flag. For now I have kept the timeout constant
as an internal detail of the inner runtest script for CoreCLR.

At this moment I haven't made any changes to the build_and_run_test
script; for some reason it didn't work in gcstress mode even before
with the environment variables. I'm going to try to investigate whether
we might be able to get rid of this duality and use the same script
for building CoreCLR CPAOT tests in both scenarios.

Thanks

Tomas

tests/CoreCLR/runtest/runtest.cmd

@@ -63,7 +63,7 @@ if /i "%1" == "PerfTests"             (set __PerfTests=true&set __SkipWrapperGen
 if /i "%1" == "runcrossgentests"      (set RunCrossGen=true&shift&goto Arg_Loop)
 REM change it to COMPlus_GCStress when we stop using xunit harness
 if /i "%1" == "gcstresslevel"         (set __GCSTRESSLEVEL=%2&set __TestTimeout=3600000&shift&shift&goto Arg_Loop)
-if /i "%1" == "LogsDir"         (set __LogsDir=%2&shift&shift&goto Arg_Loop) 
+if /i "%1" == "LogsDir"               (set __LogsDir=%2&shift&shift&goto Arg_Loop) 
 
 if /i not "%1" == "msbuildargs" goto SkipMsbuildArgs
 :: All the rest of the args will be collected and passed directly to msbuild.

tests/runtest.cmd

@@ -14,6 +14,7 @@ set CoreRT_RunCoreFXTests=
 set CoreRT_CoreCLRTargetsFile=
 set CoreRT_TestLogFileName=testResults.xml
 set CoreRT_TestName=*
+set CoreRT_GCStressLevel=
 
 :ArgLoop
 if "%1" == "" goto :ArgsDone
@@ -61,7 +62,7 @@ if /i "%1" == "/determinism" (set CoreRT_DeterminismMode=true&shift&goto ArgLoop
 if /i "%1" == "/nocleanup" (set CoreRT_NoCleanup=true&shift&goto ArgLoop)
 if /i "%1" == "/r2rframework" (set CoreRT_R2RFramework=true&shift&goto ArgLoop)
 if /i "%1" == "/user2rframework" (set CoreRT_UseR2RFramework=true&shift&goto ArgLoop)
-if /i "%1" == "/gcstresslevel" (set __GCSTRESSLEVEL=%2&set __TestTimeout=3600000&shift&shift&goto ArgLoop) 
+if /i "%1" == "/gcstresslevel" (set CoreRT_GCStressLevel=%2&shift&shift&goto ArgLoop)
 echo Invalid command line argument: %1
 goto :Usage
 
@@ -561,6 +562,7 @@ goto :eof
         call %CoreRT_TestRoot%\CoreCLR\build-and-run-test.cmd !TestFolderName! !TestFileName!
     ) else (
         set __RunTestCommand=runtest.cmd %CoreRT_BuildArch% %CoreRT_BuildType% %CoreCLRExcludeText% %CoreRT_CoreCLRTargetsFile% LogsDir %__LogDir%
+        if not "%CoreRT_GCStressLevel%" == "" ( set __RunTestCommand=!__RunTestCommand! gcstresslevel !CoreRT_GCStressLevel! )
         echo !__RunTestCommand!
         call !__RunTestCommand!
     )