[src/core/control.pm] add &sleep

It's the same as in Beijing, modulo $Inf hack.

Also, uncommented sleep.t in spectest.data.

src/core/control.pm

@@ -179,6 +179,17 @@ sub shell($cmd) {
     $status;
 }
 
+sub sleep($seconds = $Inf) {         # fractional seconds also allowed
+    my $time1 = time;
+    if $seconds ~~ $Inf {
+        pir::sleep__vN(1e16) while True;
+    } else {
+        pir::sleep__vN($seconds);
+    }
+    my $time2 = time;
+    return $time2 - $time1;
+}
+
 sub QX($cmd) {
     my Mu $pio := pir::open__Pss(nqp::unbox_s($cmd), 'rp');
     fail "Unable to execute '$cmd'" unless $pio;

t/spectest.data

@@ -468,7 +468,7 @@ S29-any/isa.t
 # S29-context/die.t # segmentation fault
 S29-context/eval.t
 S29-context/exit.t
-# S29-context/sleep.t # err: Could not find sub &sleep
+S29-context/sleep.t
 S29-conversions/hash.t
 S29-conversions/ord_and_chr.t                              #icu
 S32-array/bool.t