t/perf/speed.t: clean up code

- set @inc before loading any modules
- don't enable strict/warnings twice
- 'use Config' instead of 'require Config; Config->import'
- don't declare non-existent sub run_tests
- mention github issue number in comment in addition to RT

Author: mauke

t/perf/speed.t

@@ -11,36 +11,30 @@
 # run, rather than actually timing out.
 #
 
-use strict;
-use warnings;
-use 5.010;
-
-sub run_tests;
-
-$| = 1;
-
-
 BEGIN {
     chdir 't' if -d 't';
     @INC = ('../lib');
-    require Config; Config->import;
     require './test.pl';
 }
+use Config;
 
-plan tests => 1;
-
-use warnings;
 use strict;
+use warnings;
+use 5.010;
+
+$| = 1;
+
+plan tests => 1;
 
 watchdog(60);
 
 SKIP: {
-    # RT #121975 COW speedup lost after e8c6a474
+    # RT #121975 / GH #13878 COW speedup lost after e8c6a474
 
     # without COW, this test takes minutes; with COW, its less than a
     # second
     #
-    skip  "PERL_NO_COW", 1 if $Config{ccflags} =~ /PERL_NO_COW/;
+    skip "PERL_NO_COW", 1 if $Config{ccflags} =~ /PERL_NO_COW/;
 
     my ($x, $y);
     $x = "x" x 1_000_000;