Fix nix_api_store_test.nix_eval_state_lookup_path when run on its own

Currently, --gtest_filter=nix_api_store_test.nix_eval_state_lookup_path
will result in:

 terminating due to unexpected unrecoverable internal error: Assertion
 'gcInitialised' failed in void nix::assertGCInitialized() at
 ../src/libexpr/eval-gc.cc:138

Changing the test fixture to _exr_test causes GC to be initialised.

src/libexpr-tests/nix_api_expr.cc

@@ -14,7 +14,7 @@
 
 namespace nixC {
 
-TEST_F(nix_api_store_test, nix_eval_state_lookup_path)
+TEST_F(nix_api_expr_test, nix_eval_state_lookup_path)
 {
     auto tmpDir = nix::createTempDir();
     auto delTmpDir = std::make_unique<nix::AutoDelete>(tmpDir, true);