fuse integration: require an external mount to be a FUSE filesystem

A device-ID boundary alone does not prove FUSE: a tmpfs mount would pass mountedAt and report its results as FUSE compliance. Check the filesystem type (statfs magic on linux, fstypename on the BSDs); platforms without a check keep the mount and write verification.

test/fuse_integration/mounted_linux_test.go

@@ -0,0 +1,27 @@
+//go:build linux
+
+package fuse
+
+import "testing"
+
+func TestMountedAt(t *testing.T) {
+	for _, p := range []string{"/proc", "/proc/"} {
+		if !mountedAt(p) {
+			t.Errorf("mountedAt(%q) = false, want true", p)
+		}
+	}
+	plain := t.TempDir()
+	for _, p := range []string{plain, plain + "/"} {
+		if mountedAt(p) {
+			t.Errorf("mountedAt(%q) = true, want false", p)
+		}
+	}
+}
+
+func TestFuseFilesystem(t *testing.T) {
+	for _, p := range []string{"/proc", t.TempDir()} {
+		if fuseFilesystem(p) {
+			t.Errorf("fuseFilesystem(%q) = true, want false", p)
+		}
+	}
+}