"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

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()));
     }
 };
 

tests/functional/legacy-ssh-store.sh

@@ -1,4 +1,9 @@
 source common.sh
 
+store_uri="ssh://localhost?remote-store=$TEST_ROOT/other-store"
+
 # Check that store ping trusted doesn't yet work with ssh://
-nix --store ssh://localhost?remote-store=$TEST_ROOT/other-store store ping --json | jq -e 'has("trusted") | not'
+nix --store "$store_uri" store ping --json | jq -e 'has("trusted") | not'
+
+# Suppress grumpiness about multiple nixes on PATH
+(nix --store "$store_uri" doctor || true) 2>&1 | grep 'You are unknown trust'

tests/functional/local-store.sh

@@ -20,3 +20,6 @@ PATH3=$(nix path-info --store "local?root=$PWD/x" $CORRECT_PATH)
 
 # Ensure store ping trusted works with local store
 nix --store ./x store ping --json | jq -e '.trusted'
+
+# Suppress grumpiness about multiple nixes on PATH
+(nix --store ./x doctor || true) 2>&1 | grep 'You are trusted by'

tests/functional/remote-store.sh

@@ -13,6 +13,8 @@ startDaemon
 if isDaemonNewer "2.15pre0"; then
     # Ensure that ping works trusted with new daemon
     nix store ping --json | jq -e '.trusted'
+    # Suppress grumpiness about multiple nixes on PATH
+    (nix doctor || true) 2>&1 | grep 'You are trusted by'
 else
     # And the the field is absent with the old daemon
     nix store ping --json | jq -e 'has("trusted") | not'