Move "is test-assertion" to candidates

Apparently, it is not being seen if it just lives on the proto,
probably because the proto is not seen in callframe walking?

t/packages/Test/Helpers.pm6

@@ -48,13 +48,13 @@ sub is-run (
     }
 }
 
-proto sub is-run-repl(|) is export is test-assertion {*}
-multi sub is-run-repl($code, $out, $desc, |c) {
+proto sub is-run-repl(|) is export {*}
+multi sub is-run-repl($code, $out, $desc, |c) is test-assertion {
     is-run-repl $code, $desc, :$out, |c;
 }
 multi sub is-run-repl(
   $code is copy, $desc, :$out = '', :$err = '', :$line-editor = 'none'
-) {
+) is test-assertion {
     $code .= join: "\n" if $code ~~ Positional|Seq;
     (temp %*ENV)<RAKUDO_ERROR_COLOR  RAKUDO_LINE_EDITOR> = 0, $line-editor;
     my $proc = run $*EXECUTABLE, '--repl-mode=interactive', :in, :out, :err;
@@ -80,8 +80,10 @@ multi sub is-run-repl(
     }, $desc;
 }
 
-proto sub doesn't-hang(|) is export is test-assertion {*}
-multi sub doesn't-hang(Str $args, $desc, :$in, :$wait = 15, :$out, :$err) {
+proto sub doesn't-hang(|) is export {*}
+multi sub doesn't-hang(
+  Str $args, $desc, :$in, :$wait = 15, :$out, :$err
+) is test-assertion {
     doesn't-hang \($*EXECUTABLE.absolute, '-e', $args), $desc,
         :$in, :$wait, :$out, :$err;
 }
@@ -92,7 +94,7 @@ my $VM-time-scale-multiplier = $*VM.name eq 'jvm' ?? 20/3 !! 1;
 multi sub doesn't-hang (
     Capture $args, $desc = 'code does not hang',
     :$in, :$wait = 15, :$out, :$err,
-) {
+) is test-assertion {
     my $prog = Proc::Async.new: |$args;
     my ($stdout, $stderr) = '', '';
     $prog.stdout.tap: { $stdout ~= $^a };