App::perlbrew: use "test_harness" on perl >=5.7.3

Change the default perlbrew behavior so that it now tests via "make
test_harness" instead of "make test" if the "test_harness" target is available.

With the "test_harness" target we'll get automatic parallel testing
via $HARNESS_OPTIONS.

The + "test_harness" target was added in 5.7.3, which was the last
development release before 5.8.0.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>

lib/App/perlbrew.pm

@@ -443,7 +443,16 @@ INSTALL
             $configure_flags = '-de';
         }
 
-        my @install = $self->{notest} ? "make install" : ("make test", "make install");
+        # Test via "make test_harness" if available so we'll get
+        # automatic parallel testing via $HARNESS_OPTIONS. The
+        # "test_harness" target was added in 5.7.3, which was the last
+        # development release before 5.8.0.
+        my $test_target = "test";
+        if ($dist_version =~ /^5\.(\d+)\.(\d+)/
+            && ($1 >= 8 || $1 == 7 && $2 == 3)) {
+            $test_target = "test_harness";
+        }
+        my @install = $self->{notest} ? "make install" : ("make $test_target", "make install");
         @install    = join " && ", @install unless($self->{force});
 
         my $cmd = join ";",