Failing date format string related tests on macOS

[morgant]

When running roundup test/lib43f-*-test.sh on macOS 10.14.x, I see a lot of failing tests related to format string parsing & replacement:

  it_finds_files_with_test_file_regex:             [FAIL]
  it_finds_files_with_year_regex:                  [FAIL]
  it_replaces_percent_Y_with_year_regex:           [FAIL]
  it_replaces_percent_m_with_month_regex:          [FAIL]
  it_replaces_percent_d_with_day_regex:            [FAIL]
  it_replaces_percent_H_with_hours_regex:          [FAIL]
  it_replaces_percent_M_with_minutes_regex:        [FAIL]
  it_replaces_percent_S_with_seconds_regex:        [FAIL]
  it_replaces_percent_percent_with_percent:        [FAIL]
  it_returns_one_if_filenames_match_and_first_date_greater_than_second_date: [FAIL]
  it_returns_two_if_filenames_match_and_first_date_less_than_second_date: [FAIL]
  it_does_delete_matching_filename_with_older_datestamp: [FAIL]
  it_does_delete_multiple_matching_filenames_with_older_datestamps: [FAIL]
  it_errors_with_blank_output_format_string:       [FAIL]
  it_returns_an_empty_string_with_an_empty_date_format_string: [FAIL]
  it_does_not_modify_string_with_no_date_format_conversion_specifications: [FAIL]
  it_replaces_percent_Y_with_year_regex_in_date_format_string: [FAIL]
  it_replaces_percent_m_with_month_regex_in_date_format_string: [FAIL]
  it_replaces_percent_d_with_day_regex_in_date_format_string: [FAIL]
  it_replaces_percent_H_with_hour_regex_in_date_format_string: [FAIL]
  it_replaces_percent_M_with_minute_regex_in_date_format_string: [FAIL]
  it_replaces_percent_S_with_second_regex_in_date_format_string: [FAIL]
  it_replaces_double_percent_with_single_percent_in_date_format_string: [FAIL]
  it_replaces_multiple_conversion_specifications_in_date_format_string: [FAIL]
  it_returns_full_date_with_empty_format_string:   [FAIL]
  it_returns_empty_string_with_empty_date_string:  [FAIL]
  it_returns_empty_string_with_empty_parse_sequence_string: [FAIL]
  it_parses_four_digit_year_from_date_string:      [FAIL]
  it_parses_two_digit_month_from_date_string:      [FAIL]
  it_parses_two_digit_day_from_date_string:        [FAIL]
  it_parses_two_digit_hour_from_date_string:       [FAIL]
  it_parses_two_digit_minute_from_date_string:     [FAIL]