[NFC][flang] Fix execute_command_line test for odd environments

One of the execute_command_line tests currently runs `cat` on an invalid
file and checks its return value, but since we don't control `cat` or
the user's path, the return value might not be reliably stable on a
per-platform basis. For example, if `git` is installed on Windows in
certain configurations it adds a directory to the path containing a
`cat` with a different set of error codes to the default Windows one.

This patch changes the test to use the `not` binary built by LLVM for
testing purposes, which should always return 1 on any platform
regardless of the user's environment.

Author: DavidTruby

flang/unittests/Runtime/CommandTest.cpp

@@ -13,6 +13,8 @@
 #include "flang/Runtime/execute.h"
 #include "flang/Runtime/extensions.h"
 #include "flang/Runtime/main.h"
+#include "llvm/Support/FileSystem.h"
+#include "llvm/Support/Path.h"
 #include <cstddef>
 #include <cstdlib>
 
@@ -340,24 +342,26 @@ TEST_F(ZeroArguments, ECLValidCommandStatusSetSync) {
 }
 
 TEST_F(ZeroArguments, ECLGeneralErrorCommandErrorSync) {
-  OwningPtr<Descriptor> command{CharDescriptor("cat GeneralErrorCommand")};
+  llvm::SmallString<64> cmd;
+  if (std::error_code ec = llvm::sys::fs::current_path(cmd))
+    FAIL() << "Failed to obtain the current working directory";
+  llvm::sys::path::append(cmd, "bin", "not");
+  OwningPtr<Descriptor> command{CharDescriptor(cmd.data())};
   bool wait{true};
   OwningPtr<Descriptor> exitStat{IntDescriptor(404)};
   OwningPtr<Descriptor> cmdStat{IntDescriptor(202)};
   OwningPtr<Descriptor> cmdMsg{CharDescriptor("cmd msg buffer XXXXXXXXXXXXXX")};
 
   RTNAME(ExecuteCommandLine)
   (*command.get(), wait, exitStat.get(), cmdStat.get(), cmdMsg.get());
-#if defined(_WIN32)
   CheckDescriptorEqInt<std::int64_t>(exitStat.get(), 1);
+#if defined(_WIN32)
   CheckDescriptorEqInt<std::int64_t>(cmdStat.get(), 6);
   CheckDescriptorEqStr(cmdMsg.get(), "Invalid command lineXXXXXXXXX");
 #elif defined(_AIX)
-  CheckDescriptorEqInt<std::int64_t>(exitStat.get(), 2);
   CheckDescriptorEqInt<std::int64_t>(cmdStat.get(), 6);
   CheckDescriptorEqStr(cmdMsg.get(), "Invalid command lineXXXXXXXXX");
 #else
-  CheckDescriptorEqInt<std::int64_t>(exitStat.get(), 1);
   CheckDescriptorEqInt<std::int64_t>(cmdStat.get(), 3);
   CheckDescriptorEqStr(cmdMsg.get(), "Command line execution failed");
 #endif