runtests: use a unix domain socket path with the pid in the name

To make it impossible for test cases to access the file name without
using the proper variable for the purpose.

Closes #5264

tests/runtests.pl

@@ -5457,7 +5457,7 @@ $DICTPORT        = $base++; # DICT port
 $SMBPORT         = $base++; # SMB port
 $SMBSPORT        = $base++; # SMBS port
 $NEGTELNETPORT   = $base++; # TELNET port with negotiation
-$HTTPUNIXPATH    = 'http.sock'; # HTTP server Unix domain socket path
+$HTTPUNIXPATH    = "http$$.sock"; # HTTP server Unix domain socket path
 
 $maxport         = $base-1; # updated base port number