test.bash: don't run "go tool vet" if vendor dir exists

...this fails in a thousand ways:

[...]
vendor/golang.org/x/crypto/sha3/keccakf_amd64.s:324: [amd64] keccakF1600: unknown variable state; offset 0 is a+0(FP)
vendor/golang.org/x/crypto/ssh/certs.go:172: declaration of "err" shadows declaration at vendor/golang.org/x/crypto/ssh/certs.go:166
vendor/golang.org/x/crypto/ssh/certs.go:187: declaration of "rest" shadows declaration at vendor/golang.org/x/crypto/ssh/certs.go:161
vendor/golang.org/x/crypto/ssh/certs.go:187: declaration of "ok" shadows declaration at vendor/golang.org/x/crypto/ssh/certs.go:161
vendor/golang.org/x/crypto/ssh/client_auth.go:226: declaration of "err" shadows declaration at vendor/golang.org/x/crypto/ssh/client_auth.go:193
vendor/golang.org/x/crypto/ssh/client_auth.go:394: declaration of "err" shadows declaration at vendor/golang.org/x/crypto/ssh/client_auth.go:380
vendor/golang.org/x/crypto/ssh/client_auth.go:405: declaration of "err" shadows declaration at vendor/golang.org/x/crypto/ssh/client_auth.go:380
vendor/golang.org/x/crypto/ssh/handshake.go:566: declaration of "err" shadows declaration at vendor/golang.org/x/crypto/ssh/handshake.go:547
vendor/golang.org/x/crypto/ssh/handshake.go:592: declaration of "err" shadows declaration at vendor/golang.org/x/crypto/ssh/handshake.go:547
vendor/golang.org/x/crypto/ssh/handshake.go:630: declaration of "err" shadows declaration at vendor/golang.org/x/crypto/ssh/handshake.go:620
[...]

Author: rfjakob

test.bash

@@ -30,10 +30,12 @@ if [[ $OSTYPE == linux* ]] ; then
 	./build.bash
 fi
 
-if go tool | grep vet > /dev/null ; then
-	go tool vet -all -shadow .
-else
+if ! go tool | grep vet > /dev/null ; then
 	echo "'go tool vet' not available - skipping"
+elif [[ -d vendor ]] ; then
+	echo "vendor directory exists, skipping 'go tool vet'"
+else
+	go tool vet -all -shadow .
 fi
 
 # We don't want all the subprocesses holding the lock file open