Move unit tests to the location Meson expects them to be

Everything that is a separate subproject should live in the subprojects
directory.

Progress on #2503

This reverts commit 451f8a8c19.

src/libstore-tests/legacy-ssh-store.cc

@@ -0,0 +1,26 @@
+#include <gtest/gtest.h>
+
+#include "legacy-ssh-store.hh"
+
+namespace nix {
+
+TEST(LegacySSHStore, constructConfig)
+{
+    LegacySSHStoreConfig config{
+        "ssh",
+        "localhost",
+        StoreConfig::Params{
+            {
+                "remote-program",
+                // TODO #11106, no more split on space
+                "foo bar",
+            },
+        }};
+    EXPECT_EQ(
+        config.remoteProgram.get(),
+        (Strings{
+            "foo",
+            "bar",
+        }));
+}
+}