Fix: use threadsafe death tests in DFPT suites to avoid fork-in-threaded-process deadlock

MODULE_DFPT_pw_run_test timed out (1700 s) in the gnu Test CI job: the
two irrep-loop tests run first execute OpenMP regions, so with the job's
OMP_NUM_THREADS=2 the process is multithreaded when the third test
(dftu-reservation EXPECT_EXIT) forks. The default fast-style child then
deadlocks on exit and the parent waits forever (reproduced locally under
OMP_NUM_THREADS=2: gtest warns 'detected 2 threads' and hangs).

Switch all three DFPT death tests to the fork+exec threadsafe style
(same pattern as module_container tensor_test). For the pw_run test also
bridge std::cout to std::cerr inside the death statement: WARNING_QUIT
prints the NOTICE block to stdout, while death tests match the child's
stderr; the old CaptureStdout+HasSubstr assertion cannot see the re-exec
child's output. Verified under OMP_NUM_THREADS=2: pw_run 3/3 in 0.3 s
(previously indefinite hang), kq_basis 5/5, pert_serial 8/8, and the
full MODULE_DFPT ctest batch 8/8.

Author: Zanthoxylum

source/source_pw/module_dfpt/test/dfpt_kq_basis_test.cpp

@@ -383,6 +383,7 @@ TEST_F(DFPTKQBasisTest, TranslationInvarianceOfKQ)
 
 TEST_F(DFPTKQBasisTest, InvalidOrMismatchedBaseIsRejected)
 {
+    ::testing::FLAGS_gtest_death_test_style = "threadsafe";
     // null providers: valid-but-empty basis
     ModuleDFPT::DFPT_KQ_Basis kq;
     kq.init(nullptr, nullptr, ModuleBase::Vector3<double>(0.0, 0.0, 0.0), 0);

source/source_pw/module_dfpt/test/dfpt_pw_run_test.cpp

@@ -214,6 +214,10 @@ TEST_F(DFPT_PWRunTest, DielectricAndBornAreExposed)
 
 TEST_F(DFPT_PWRunTest, DftuReservationWithProviderRejectsInit)
 {
+    // the preceding irrep-loop tests run OpenMP regions, so the default
+    // "fast" fork-based death test can deadlock in the multithreaded child
+    // (CI: OMP_NUM_THREADS=2); use the fork+exec style instead
+    ::testing::FLAGS_gtest_death_test_style = "threadsafe";
     // DFT+U reservation (U0): the ground state wires a provider when
     // dft_plus_u is enabled upstream, and PW-basis DFT+U actually runs in
     // the ground state now. Since every DFPT U hook (cal_docc, build_dv_u,
@@ -227,11 +231,11 @@ TEST_F(DFPT_PWRunTest, DftuReservationWithProviderRejectsInit)
     Plus_U_Base dftu;
     dfpt.set_qmesh(1, 1, 1);
     psi::Psi<std::complex<double>> psi;
-    testing::internal::CaptureStdout();
-    EXPECT_EXIT(dfpt.init(ucell, psi, nullptr, nullptr, nullptr, std::vector<double>(),
-                          ModuleBase::matrix(), ModuleBase::matrix(), nullptr, 1.0, 15.0, &dftu),
-                ::testing::ExitedWithCode(1),
-                "");
-    const std::string output = testing::internal::GetCapturedStdout();
-    EXPECT_THAT(output, testing::HasSubstr("DFT+U with DFPT is not supported"));
+    // death tests match the child's stderr, while WARNING_QUIT writes the
+    // NOTICE block to std::cout; bridge the two inside the statement
+    EXPECT_EXIT({
+        std::cout.rdbuf(std::cerr.rdbuf());
+        dfpt.init(ucell, psi, nullptr, nullptr, nullptr, std::vector<double>(),
+                  ModuleBase::matrix(), ModuleBase::matrix(), nullptr, 1.0, 15.0, &dftu);
+    }, ::testing::ExitedWithCode(1), "DFT\\+U with DFPT is not supported");
 }

source/source_pw/module_dfpt/test_serial/dfpt_pert_serial_test.cpp

@@ -624,6 +624,7 @@ TEST_F(DFPTPertSerialTest, DVnlDtauMatchesOperatorFiniteDifference)
 
 TEST_F(DFPTPertSerialTest, NonlocalPathRejectsUltrasoft)
 {
+    ::testing::FLAGS_gtest_death_test_style = "threadsafe";
     MakeNCAtom();
     ucell_.atoms[0].ncpp.tvanp = true;
     const int npwk = pw_wfc_.npwk[0];