Smartify harness automatic job number calculation

- Use TEST_JOBS if we were given any
- Instead of using default 6 jobs, calculate using the number of CPUs
    on the box. Multiplying number by 1.3 is the optimal number of
    test jobs I found a while back.
- On harness6 old default was 1 by mistake; it was supposed to be 6
- Default to 6 cores if our cpu-core computation fails for some reason

t/harness5

@@ -24,7 +24,7 @@ GetOptions(
     'tests-from-file=s' => \my $list_file,
     'fudge'             => \my $do_fudge,
     'verbosity=i'       => \$Test::Harness::verbose,
-    'jobs:1'            => \(my $jobs = $ENV{TEST_JOBS} || 6),
+    'jobs=i'            => \my $jobs,
     'quick:1'           => \my $do_quick,
     'stress:1'          => \my $do_stress,
     'archive=s'         => \my $archive,
@@ -39,8 +39,13 @@ GetOptions(
 my @pass_through_options = grep m/^--?[^-]/, @ARGV;
 my @files = grep m/^[^-]/, @ARGV;
 
-$ENV{'HARNESS_PERL'} = ".${slash}perl6-" . ($moar ? "m" : $jvm ? "j" : "m");
-$ENV{'PERL6LIB'} = "lib";
+$ENV{HARNESS_PERL} = ".${slash}perl6-" . ($moar ? "m" : $jvm ? "j" : "m");
+$ENV{PERL6LIB} = "lib";
+
+$jobs ||= $ENV{TEST_JOBS}
+      || sprintf("%d", 1.3 * qx/$ENV{HARNESS_PERL} -e Kernel.cpu-cores.print/)
+      || 6;
+print "Using $jobs test jobs\n";
 
 my @slow;
 if ($list_file) {

t/harness6

@@ -29,7 +29,7 @@ sub MAIN(
     Str  :$tests-from-file is copy = Str,
     Bool :$fudge = False,
     Int  :$verbosity = (%*ENV<TEST_VERBOSE> // 0).Int,
-    Int  :$jobs = (%*ENV<TEST_JOBS> // 1).Int,
+    Int  :$jobs = (%*ENV<TEST_JOBS> || Int(1.3 * Kernel.cpu-cores) || 6).Int,
     Bool :$quick = False,
     Bool :$stress = False,
     Bool :$randomize = False,
@@ -38,6 +38,7 @@ sub MAIN(
     Str  :$perl5path = 'perl',
     *@files,
 ) {
+    say "Using $jobs test jobs";
     my @slow;
     with $tests-from-file {
         $tests-from-file .= &convert-to-versioned-file;