tests: make it possible to set executable extensions

This enables the use of Windows Subsystem for Linux (WSL) to run the
testsuite against Windows binaries while using Linux servers.

This commit introduces the following environment variables:
- CURL_TEST_EXE_EXT: set the executable extension for all components
- CURL_TEST_EXE_EXT_TOOL: set it for the curl tool only
- CURL_TEST_EXE_EXT_SSH: set it for the SSH tools only

Later testcurl.pl could be adjusted to make use of those variables.
- CURL_TEST_EXE_EXT_SRV: set it for the test servers only

(This is one of several commits to support use of WSL for the tests.)

Closes https://github.com/curl/curl/pull/3899

tests/ftpserver.pl

@@ -68,6 +68,10 @@ use serverhelp qw(
     datasockf_logfilename
     );
 
+use sshhelp qw(
+    exe_ext
+    );
+
 #**********************************************************************
 # global vars...
 #
@@ -411,7 +415,7 @@ sub sysread_or_die {
 }
 
 sub startsf {
-    my $mainsockfcmd = "./server/sockfilt " .
+    my $mainsockfcmd = "./server/sockfilt".exe_ext('SRV')." " .
         "--ipv$ipvnum --port $port " .
         "--pidfile \"$mainsockf_pidfile\" " .
         "--logfile \"$mainsockf_logfile\"";
@@ -2401,7 +2405,7 @@ sub PASV_ftp {
     logmsg "DATA sockfilt for passive data channel starting...\n";
 
     # We fire up a new sockfilt to do the data transfer for us.
-    my $datasockfcmd = "./server/sockfilt " .
+    my $datasockfcmd = "./server/sockfilt".exe_ext('SRV')." " .
         "--ipv$ipvnum $bindonly --port 0 " .
         "--pidfile \"$datasockf_pidfile\" " .
         "--logfile \"$datasockf_logfile\"";
@@ -2620,7 +2624,7 @@ sub PORT_ftp {
     logmsg "DATA sockfilt for active data channel starting...\n";
 
     # We fire up a new sockfilt to do the data transfer for us.
-    my $datasockfcmd = "./server/sockfilt " .
+    my $datasockfcmd = "./server/sockfilt".exe_ext('SRV')." " .
         "--ipv$ipvnum --connect $port --addr \"$addr\" " .
         "--pidfile \"$datasockf_pidfile\" " .
         "--logfile \"$datasockf_logfile\"";