runtests: fix output to command log

- Record only the command of the most recently ran test in the command
  log.

This is a follow-up to 02988b7 from several weeks ago which fixed
writing to the command log, however it saved all commands for all tests
instead of just the most recently ran test as we would now expect.

Fixes https://github.com/curl/curl/commit/02988b7#commitcomment-37546876
Closes https://github.com/curl/curl/pull/5001

tests/runtests.pl

@@ -2566,7 +2566,7 @@ sub cleardir {
     opendir(DIR, $dir) ||
         return 0; # can't open dir
     while($file = readdir(DIR)) {
-        if(($file !~ /^\./) && ($file ne $CURLLOG)) {
+        if(($file !~ /^\./)) {
             unlink("$dir/$file");
             $count++;
         }
@@ -3721,7 +3721,9 @@ sub singletest {
         logmsg "$CMDLINE\n";
     }
 
+    open(CMDLOG, ">", "$LOGDIR/$CURLLOG");
     print CMDLOG "$CMDLINE\n";
+    close(CMDLOG);
 
     unlink("core");
 
@@ -5410,14 +5412,6 @@ if($scrambleorder) {
     $TESTCASES = join(" ", @rand);
 }
 
-#######################################################################
-# Start the command line log
-#
-open(CMDLOG, ">", "$LOGDIR/$CURLLOG") ||
-    logmsg "can't log command lines to $CURLLOG\n";
-
-#######################################################################
-
 # Display the contents of the given file.  Line endings are canonicalized
 # and excessively long files are elided
 sub displaylogcontent {
@@ -5583,11 +5577,6 @@ if(azure_check_environment() && $AZURE_RUN_ID) {
     $AZURE_RUN_ID = azure_update_test_run($AZURE_RUN_ID);
 }
 
-#######################################################################
-# Close command log
-#
-close(CMDLOG);
-
 # Tests done, stop the servers
 stopservers($verbose);