libphobos: Fix instability in the parallelized testsuite

libphobos.unittest/unittest.exp calls bare dg-test rather than dg-runtest, and
so it should call runtest_file_p to determine whether to run each test or
not. Without that call, the tests run too many times in parallel mode (they will
run as many times, as the argument to make -j).

libphobos/ChangeLog:

	* testsuite/libphobos.unittest/unittest.exp: Call runtest_file_p
	prior to running each test.

diff --git a/libphobos/testsuite/libphobos.unittest/unittest.exp b/libphobos/testsuite/libphobos.unittest/unittest.exp
index 2a019caca8c..175decdc333 100644
--- a/libphobos/testsuite/libphobos.unittest/unittest.exp
+++ b/libphobos/testsuite/libphobos.unittest/unittest.exp
@@ -42,6 +42,9 @@ foreach unit_test $unit_test_list {
     set expected_fail [lindex $unit_test 1]
 
     foreach test $tests {
+	if {![runtest_file_p $runtests $test]} {
+            continue
+        }
         set shouldfail $expected_fail
         dg-test $test "" $test_flags
     }