Ensure we can construct remote store configs in isolation

Progress towards #10766

I thought that #10768 achieved, but when I went to use this stuff (in
Hydra), turns out it did not. (Those `using FooConfig;` lines were not
working --- they are so finicky!) This PR gets the job done, and adds
some trivial unit tests to make sure I did what I intended.

I had to add add a header to expose `SSHStoreConfig`, after which the
preexisting `ssh-store-config.*` were very confusingly named files, so I
renamed them to `common-ssh-store-config.hh` to match the type defined
therein.

tests/unit/libstore/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",
+        }));
+}
+}

tests/unit/libstore/meson.build

@@ -58,6 +58,7 @@ sources = files(
   'derivation.cc',
   'derived-path.cc',
   'downstream-placeholder.cc',
+  'legacy-ssh-store.cc',
   'machines.cc',
   'nar-info-disk-cache.cc',
   'nar-info.cc',
@@ -67,6 +68,7 @@ sources = files(
   'path.cc',
   'references.cc',
   'serve-protocol.cc',
+  'ssh-store.cc',
   'store-reference.cc',
   'worker-protocol.cc',
 )

tests/unit/libstore/ssh-store.cc

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