Tests: close the .hkl before removing it; floor the GPU model-scale B tolerance

The SHELX .hkl test removed its file while its own ifstream still held it
open, which Windows refuses. The GPU-vs-gemmi model-scale check held b_star
to 1e-4 of its largest element alone, which on an F4132 case fitted at
B 0.6 A^2 failed on a 1e-4 A^2 difference; it now also allows 0.001 A^2
of B.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K5K8jvPPbmCrbqnWkddTuB

tests/WriteReflectionsTest.cpp

@@ -200,10 +200,13 @@ TEST_CASE("SHELX .hkl of unmerged fulls: fixed 3I4,2F8.2, scaled to fit, 0 0 0 t
     const auto path = (std::filesystem::temp_directory_path() / "jfjoch_scaled_fulls_test.hkl").string();
     WriteShelxHklReflections(fulls, path, 2);
 
-    std::ifstream in(path);
     std::vector<std::string> lines;
-    for (std::string line; std::getline(in, line);)
-        lines.push_back(line);
+    {
+        // Closed before the remove: Windows refuses to delete a file that is still open.
+        std::ifstream in(path);
+        for (std::string line; std::getline(in, line);)
+            lines.push_back(line);
+    }
     std::filesystem::remove(path);
 
     REQUIRE(lines.size() == 4);