[NFC][tsan] Move variadic-open test

There is nothing Darwin or tsan specific in the test.

For #108291

Author: vitalybuka

compiler-rt/test/tsan/Darwin/variadic-open.cpp renamed to compiler-rt/test/sanitizer_common/TestCases/Posix/variadic-open.cpp

@@ -1,4 +1,5 @@
-// RUN: %clangxx_tsan -O1 %s -o %t && %run %t %t.tmp 2>&1 | FileCheck %s
+// RUN: %clangxx -O1 %s -o %t && %run %t %t.tmp 2>&1 | FileCheck %s
+
 #include <stdio.h>
 #include <assert.h>
 #include <fcntl.h>