Allow unit test infra to be reused across libs' tests

This allows using Arbitrary "instances" defined in libstore-tests in
libexpr-tests, something we will leverage in a moment.

src/libstore/tests/local.mk

@@ -1,6 +1,20 @@
-check: libstore-tests_RUN
+check: libstore-tests-exe_RUN
 
-programs += libstore-tests
+programs += libstore-tests-exe
+
+libstore-tests-exe_NAME = libnixstore-tests
+
+libstore-tests-exe_DIR := $(d)
+
+libstore-tests-exe_INSTALL_DIR :=
+
+libstore-tests-exe_LIBS = libstore-tests
+
+libstore-tests-exe_LDFLAGS := $(GTEST_LIBS)
+
+libraries += libstore-tests
+
+libstore-tests_NAME = libnixstore-tests
 
 libstore-tests_DIR := $(d)
 

src/libstore/tests/path.cc

@@ -7,7 +7,8 @@
 #include "path-regex.hh"
 #include "store-api.hh"
 
-#include "libstoretests.hh"
+#include "tests/libstore.hh"
+#include "tests/path.hh"
 
 namespace nix {
 
@@ -73,11 +74,6 @@ void showValue(const StorePath & p, std::ostream & os) {
 namespace rc {
 using namespace nix;
 
-template<>
-struct Arbitrary<StorePath> {
-    static Gen<StorePath> arbitrary();
-};
-
 Gen<StorePath> Arbitrary<StorePath>::arbitrary()
 {
     auto len = *gen::inRange<size_t>(1, StorePath::MaxPathLen);

src/libstore/tests/path.hh

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <rapidcheck.h>
+
+#include "path.hh"
+
+namespace rc {
+using namespace nix;
+
+template<>
+struct Arbitrary<StorePath> {
+    static Gen<StorePath> arbitrary();
+};
+
+}