tests: make it possible to set executable extensions

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