tests/template: Make scripts for curl, wget, fetch

Turns out the 'fail to find tar uses git clone' test would fail on
systems that don't have `curl` installed, since
`create_forwarding_script` now only creates a script if a program is
present. This fixes the breakage by trying to create a forwarding script
for all three programs; at least one should be guaranteed to be present.

Author: mbland

tests/template.bats

@@ -335,7 +335,10 @@ run_with_download_program() {
 }
 
 @test "$SUITE: fail to find tar uses git clone" {
+  # If none of these are installed on the system, the test will fail.
   create_forwarding_script 'curl'
+  create_forwarding_script 'wget'
+  create_forwarding_script 'fetch'
   PATH="$BATS_TEST_BINDIR" run "$BASH" "$TEST_GO_ROOTDIR/go-template"
 
   assert_output_matches "Failed to find tar"