"but doctor, I AM the untrusted store": nix doctor had wrong trustedness

This probably snuck in in a refactor using truthiness or so. The
trustedness flag was having the optional fullness checked, rather than
the actual contained trust level.

Also adds some tests.

```
m1@6876551b-255d-4cb0-af02-8a4f17b27e2e ~ % nix store ping
warning: 'nix store ping' is a deprecated alias for 'nix store info'
Store URL: daemon
Version: 2.20.4
Trusted: 0
m1@6876551b-255d-4cb0-af02-8a4f17b27e2e ~ % nix doctor
warning: 'doctor' is a deprecated alias for 'config check'
[PASS] PATH contains only one nix version.
[PASS] All profiles are gcroots.
[PASS] Client protocol matches store protocol.
[INFO] You are trusted by store uri: daemon
```

Fixes: https://git.lix.systems/lix-project/lix/issues/232
Change-Id: I21576e2a0a755036edf8814133345987617ba3d0

1 files changed, 3 insertions, 4 deletions

diff --git a/src/nix/doctor.cc b/src/nix/doctor.cc
index 531f0ad86..da7a1d7a0 100644
--- a/src/nix/doctor.cc
+++ b/src/nix/doctor.cc
@@ -146,10 +146,9 @@ struct CmdDoctor : StoreCommand
 
     void checkTrustedUser(ref<Store> store)
     {
-        std::string_view trusted = store->isTrustedClient()
-            ? "trusted"
-            : "not trusted";
-        checkInfo(fmt("You are %s by store uri: %s", trusted, store->getUri()));
+        auto trustedMay = store->isTrustedClient();
+        std::string_view trustedness = trustedMay ? (*trustedMay ? "trusted" : "not trusted") : "unknown trust";
+        checkInfo(fmt("You are %s by store uri: %s", trustedness, store->getUri()));
     }
 };