This PR refines the handling of status source code locations and overhauls related tests. The key changes improve the accuracy of source information and streamline our testing approach:

• Resolved an undesirable behavior where the source location of every status propagation point (macro usage) was appended to the error message, effectively creating an unwanted backtrace. Now, source location information is only appended when the message is explicitly overwritten with a more informative one, providing relevant context without clutter.
• Added nested error propagation test MacroReturnIfErrorWithNestedError that exemplifies the undesirable behavior.
• Removed test_status.cpp, copying its tests to the other 2 existing tests: test_status_show_cpp_error_context.cpp and test_status_dont_show_cpp_error_context.cpp. Rationale: the tests should pass with both configurations.
• Added a reminder on each test file, so that whenever one test is added to one of them, it's also added to the other.

Undesired Behavior

Below, we can see the results of the newly added test MacroReturnIfErrorWithNestedError when XLA_SHOW_CPP_ERROR_CONTEXT=1:

• Expected: error message with source code location
• Actual: error message with soruce code location of all the places we've propagated the status

TEST(StatusWithErrorContextTest, MacroReturnIfErrorWithNestedError) {
  auto test_function = []() -> Status {
    Status error_status = absl::InvalidArgumentError(message);
    XLA_RETURN_IF_ERROR(error_status);
    return absl::OkStatus();
  };

  auto outer_test_function = [&]() -> Status {
    XLA_RETURN_IF_ERROR(test_function());
    return absl::OkStatus();
  };

  Status result = outer_test_function();
}

[ RUN      ] StatusWithErrorContextTest.MacroReturnIfErrorWithNestedError
test/cpp/test_status_show_cpp_error_context.cpp:132: Failure
Expected equality of these values:
  result.message()
    Which is: "Test error message (at test/cpp/test_status_show_cpp_error_context.cpp:116) (at test/cpp/test_status_show_cpp_error_context.cpp:120) (at test/cpp/test_status_show_cpp_error_context.cpp:125)"
  StrCat("Test error message (at ", "test/cpp/test_status_show_cpp_error_context.cpp", ":", errline, ")")
    Which is: "Test error message (at test/cpp/test_status_show_cpp_error_context.cpp:116)"

[  FAILED  ] StatusWithErrorContextTest.MacroReturnIfErrorWithNestedError (0 ms)