Make t/harness* use 6 TEST_JOBS by default

Many of installations instructions fail to specify the need to
use TEST_JOBS (or users may miss them), which causes the spectest
to take over half an hour to run.

Default the jobs to 6, which is a more suitable default for many
of the today's common boxes.

INSTALL.txt

@@ -10,7 +10,7 @@
     (Perl is installed by default already). To enable parallel testing you
     also need the CPAN module Test::Harness in version 3.16 or newer; you
     can control the number of parallel jobs with the "TEST_JOBS" environment
-    variable.
+    variable. If TEST_JOBS is not specified, 6 jobs will be used.
 
   Building and invoking Rakudo
     If you're wanting the bleeding-edge version of the Rakudo Perl 6

t/harness5

@@ -22,7 +22,7 @@ GetOptions(
     'tests-from-file=s' => \my $list_file,
     'fudge'             => \my $do_fudge,
     'verbosity=i'       => \$Test::Harness::verbose,
-    'jobs:1'            => \(my $jobs = $ENV{TEST_JOBS} || 1),
+    'jobs:1'            => \(my $jobs = $ENV{TEST_JOBS} || 6),
     'quick:1'           => \my $do_quick,
     'stress:1'          => \my $do_stress,
     'archive=s'         => \my $archive,

t/harness6

@@ -26,7 +26,7 @@ multi sub MAIN(
     Str  :$tests-from-file = Str,
     Bool :$fudge = False,
     Int  :$verbosity = (%*ENV<TEST_VERBOSE> // 0).Int,
-    Int  :$jobs = (%*ENV<TEST_JOBS> // 1).Int,
+    Int  :$jobs = (%*ENV<TEST_JOBS> // 6).Int,
     Bool :$quick = False,
     Bool :$stress = False,
     Bool :$randomize = False,