[libcxx] [test] Mark tests that require specific allocation behaviours as libcpp only

This fixes/silences a few failures on libstdc++ on linux.

Differential Revision: https://reviews.llvm.org/D89676

Author: mstorsjo

libcxx/test/std/input.output/filesystems/class.path/path.member/path.append.pass.cpp

@@ -331,8 +331,8 @@ int main(int, char**)
     doAppendSourceTest<char32_t>(TC);
   }
   for (auto const & TC : LongLHSCases) {
-    doAppendSourceAllocTest<char>(TC);
-    doAppendSourceAllocTest<wchar_t>(TC);
+    LIBCPP_ONLY(doAppendSourceAllocTest<char>(TC));
+    LIBCPP_ONLY(doAppendSourceAllocTest<wchar_t>(TC));
   }
   test_sfinae();
 

libcxx/test/std/input.output/filesystems/class.path/path.member/path.concat.pass.cpp

@@ -356,7 +356,7 @@ int main(int, char**)
       const char* E = TC.expect;
       PathReserve(LHS, StrLen(E) + 5);
       {
-        DisableAllocationGuard g;
+        LIBCPP_ONLY(DisableAllocationGuard g);
         path& Ref = (LHS += RHS);
         assert(&Ref == &LHS);
       }
@@ -368,14 +368,14 @@ int main(int, char**)
       const char* E = TC.expect;
       PathReserve(LHS, StrLen(E) + 5);
       {
-        DisableAllocationGuard g;
+        LIBCPP_ONLY(DisableAllocationGuard g);
         path& Ref = (LHS += RHS);
         assert(&Ref == &LHS);
       }
       assert(LHS == E);
     }
-    doConcatSourceAllocTest<char>(TC);
-    doConcatSourceAllocTest<wchar_t>(TC);
+    LIBCPP_ONLY(doConcatSourceAllocTest<char>(TC));
+    LIBCPP_ONLY(doConcatSourceAllocTest<wchar_t>(TC));
   }
   for (auto const& TC : CharTestCases) {
     doConcatECharTest<char>(TC);

libcxx/test/std/input.output/filesystems/class.path/path.member/path.native.obs/string_alloc.pass.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03
+// REQUIRES: libc++
 
 // <filesystem>