[libcxx] [test] Fix hard_link_count test to account for fs with dir nlink==1

[mgorny]

Summary

Filesystems are not required to maintain a hard link count consistent
with number of subdirectories. For example, on btrfs all directories
have nlink==1. Account for that in the test.

libcxx/trunk/test/std/experimental/filesystem/fs.op.funcs/fs.op.hard_lk_ct/hard_link_count.pass.cpp

 #if defined(__APPLE__)
     // Filesystems formatted with case sensitive hfs+ behave unixish as
     // expected. Normal hfs+ filesystems report the number of directory
     // entries instead.
     DirExpectAlt = 5; // .  ..  Dir2  file1  file2
     Dir3Expect = 3; // .  ..  file5
 #endif
     TEST_CHECK(hard_link_count(StaticEnv::Dir) == DirExpect ||
-               hard_link_count(StaticEnv::Dir) == DirExpectAlt);
+               hard_link_count(StaticEnv::Dir) == DirExpectAlt ||
+               hard_link_count(StaticEnv::Dir) == 1);
     TEST_CHECK(hard_link_count(StaticEnv::Dir3) == Dir3Expect ||
-               hard_link_count(StaticEnv::Dir3) == Dir3ExpectAlt);
+               hard_link_count(StaticEnv::Dir3) == Dir3ExpectAlt ||
+               hard_link_count(StaticEnv::Dir3) == 1);
 
     std::error_code ec;
     TEST_CHECK(hard_link_count(StaticEnv::Dir, ec) == DirExpect ||
-               hard_link_count(StaticEnv::Dir, ec) == DirExpectAlt);
+               hard_link_count(StaticEnv::Dir, ec) == DirExpectAlt ||
+               hard_link_count(StaticEnv::Dir) == 1);
     TEST_CHECK(hard_link_count(StaticEnv::Dir3, ec) == Dir3Expect ||
-               hard_link_count(StaticEnv::Dir3, ec) == Dir3ExpectAlt);
+               hard_link_count(StaticEnv::Dir3, ec) == Dir3ExpectAlt ||
+               hard_link_count(StaticEnv::Dir3) == 1);
 }
 TEST_CASE(hard_link_count_increments_test)
 {
     scoped_test_env env;
     const path file = env.create_file("file", 42);
     TEST_CHECK(hard_link_count(file) == 1);
 
     env.create_hardlink(file, "file_hl");