Make the file_test binary work without custom environment variables. (#5442)

Instead of crashing when run outside of `bazel`, make the toolchain's
`file_test` binary work properly when no test-specific environment
variables are set. This makes it a lot easier to run `file_test` under a
debugger.

There are two main changes here:

- Don't crash if `$TEST_TMPDIR` is unset. Instead, fall back to LLVM's
temporary directory (typically `$TMPDIR`). We already did this in some
places in tests. We now do it in more places.
- Don't fall back to a target label of `<target>` in the reproduction
commands if `$TEST_TARGET` is unset, because this causes all the tests
to fail because their output doesn't match the expected output due to a
differing bazel run command. Instead explicitly specify the target from
the `FileTestBase`-derived class.

Infrastructure for this has been added generally, but only rolled out to
the toolchain `file_test` binary for now.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

testing/file_test/run_test.cpp

@@ -14,6 +14,7 @@
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/PrettyStackTrace.h"
+#include "testing/base/file_helpers.h"
 #include "testing/file_test/file_test_base.h"
 #include "testing/file_test/test_file.h"
 
@@ -62,8 +63,7 @@ static auto DoArgReplacements(llvm::SmallVector<std::string>& test_args,
         break;
       }
       case 't': {
-        char* tmpdir = getenv("TEST_TMPDIR");
-        CARBON_CHECK(tmpdir != nullptr);
+        std::filesystem::path tmpdir = GetTempDirectory();
         it->replace(percent, 2, llvm::formatv("{0}/temp_file", tmpdir));
         break;
       }