Fix external dependency (#641)

By putting it in here. Closes #640.

S26-documentation/block-leading-user-format.t

@@ -1,7 +1,5 @@
 use v6;
 use Test;
-use lib <t/packages>;
-use Test::Helpers;
 
 plan 2;
 
@@ -74,3 +72,37 @@ for %h.keys -> $s {
 
 
 }
+
+sub is-run (
+    Str() $code, $desc = "$code runs",
+    Stringy :$in, :@compiler-args, :@args, :$out = '', :$err = '', :$exitcode = 0
+) is export {
+    my @proc-args = flat do if $*DISTRO.is-win {
+        # $*EXECUTABLE is a batch file on Windows, that goes through cmd.exe
+        # and chokes on standard quoting. We also need to remove any newlines
+        <cmd.exe  /S /C>, $*EXECUTABLE, @compiler-args, '-e',
+        ($code,  @args).subst(:g, "\n", " ")
+    }
+    else {
+        $*EXECUTABLE, @compiler-args, '-e', $code, @args
+    }
+
+    with run :in, :out, :err, @proc-args {
+        $in ~~ Blob ?? .in.write: $in !! .in.print: $in if $in;
+        $ = .in.close;
+        my $proc-out      = .out.slurp: :close;
+        my $proc-err      = .err.slurp: :close;
+        my $proc-exitcode = .exitcode;
+
+        my $wanted-exitcode = $exitcode // 0;
+        my $wanted-out      = $out    // '';
+        my $wanted-err      = $err    // '';
+
+        subtest $desc => {
+            plan 3;
+            cmp-ok $proc-out,      '~~', $wanted-out,      'STDOUT';
+            cmp-ok $proc-err,      '~~', $wanted-err,      'STDERR';
+            cmp-ok $proc-exitcode, '~~', $wanted-exitcode, 'Exit code';
+        }
+    }
+}