Fix various segfaults when running go test manually (#8448)

* failing to find i18n shouldn't segfault

The server was trying to handle the fact that it couldn't find the i18n
directory, by emitting a translated log message...

* fix utils.FindDir

The attempts to find the directory in the parent or grandparent directory
don't work if the current working directory was inside `enterprise`, with
`enterprise` itself being a symlink as per the usual developer setup.

Recurse to the root of the filesystem, cleaning the path along the way
to work around this limitation (and allow tests to be run from an
arbitrarily deep nesting level.)

Fix corresponding usages to employ filepath.Join.

* failing to find html templates shouldn't segfault

* fail fast if the test user cannot be created

* rework utils.FindDir to retain backwards compatibility

api/file_test.go

@@ -10,6 +10,7 @@ import (
 	"io/ioutil"
 	"net/http"
 	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 	"time"
@@ -870,7 +871,7 @@ func TestGetInfoForFilename(t *testing.T) {
 
 func readTestFile(name string) ([]byte, error) {
 	path, _ := utils.FindDir("tests")
-	file, err := os.Open(path + "/" + name)
+	file, err := os.Open(filepath.Join(path, name))
 	if err != nil {
 		return nil, err
 	}

api/user_test.go

@@ -12,6 +12,7 @@ import (
 	"mime/multipart"
 	"net/http"
 	"os"
+	"path/filepath"
 	"strings"
 	"testing"
 	"time"
@@ -778,7 +779,7 @@ func TestUserUploadProfileImage(t *testing.T) {
 		}
 
 		path, _ := utils.FindDir("tests")
-		file, err := os.Open(path + "/test.png")
+		file, err := os.Open(filepath.Join(path, "test.png"))
 		if err != nil {
 			t.Fatal(err)
 		}