REGTESTS: Replace REQUIRE_BINARIES with 'command -v'

This migrates the tests to the native `feature cmd` functionality of VTest.
diff --git a/reg-tests/ssl/set_ssl_cafile.vtc b/reg-tests/ssl/set_ssl_cafile.vtc
index 5dcfaf9..72ce3e6 100644
--- a/reg-tests/ssl/set_ssl_cafile.vtc
+++ b/reg-tests/ssl/set_ssl_cafile.vtc
@@ -17,7 +17,7 @@
 varnishtest "Test the 'set ssl ca-file' feature of the CLI"
 feature cmd "$HAPROXY_PROGRAM -cc 'version_atleast(2.5-dev0)'"
 feature cmd "$HAPROXY_PROGRAM -cc 'feature(OPENSSL)'"
-#REQUIRE_BINARIES=socat
+feature cmd "command -v socat"
 feature ignore_unknown_macro
 
 server s1 -repeat 4 {