input: Write a python function to extract bold strings from an Excel file.
  assert extract_bold_strings('test1.xlsx') == ['Bold1', 'Bold2']
  assert extract_bold_strings('test2.xlsx') == ['BoldText1']
  assert extract_bold_strings('test3.xlsx') == []
output: def extract_bold_strings(file_path):
      from openpyxl import load_workbook
      from openpyxl.styles import Font
  
      bold_strings = []
      wb = load_workbook(file_path)
      for sheet in wb.worksheets:
          for row in sheet.iter_rows():
              for cell in row:
                  if cell.font and cell.font.bold:
                      bold_strings.append(cell.value)
      return bold_strings