Fix fs::proximate tests on platforms where /net exists.

The proximate tests depended on `/net` not being a valid path,
however, on OS X it is.

Correct the tests to handle this.

llvm-svn: 329038

libcxx/test/std/experimental/filesystem/fs.op.funcs/fs.op.proximate/proximate.pass.cpp

@@ -79,10 +79,10 @@ TEST_CASE(basic_test) {
       {"a", "/", relative_cwd / "a"},
       {"a/b", "/", relative_cwd / "a/b"},
       {"a", "/net", ".." / relative_cwd / "a"},
-      {"//net/", "//net", "/net/"},
-      {"//net", "//net/", ".."},
-      {"//net", "//net", "."},
-      {"//net/", "//net/", "."},
+      {"//foo/", "//foo", "/foo/"},
+      {"//foo", "//foo/", ".."},
+      {"//foo", "//foo", "."},
+      {"//foo/", "//foo/", "."},
       {"//base", "a", dot_dot_to_root / "../base"},
       {"a", "a", "."},
       {"a/b", "a/b", "."},