TST: move Styler format tests to own module
The following tests moved from `style/test_style.py` to their own module `style/test_format.py`.

def test_display_format(styler):
def test_format_dict(styler):
def test_format_string(styler):  [added new]
def test_format_callable(styler):  [added new]
def test_format_with_na_rep():
def test_format_non_numeric_na():
def test_format_clear(styler):
def test_format_escape():
def test_format_escape_na_rep():
def test_format_escape_floats(styler):
def test_format_raises(styler, formatter):
def test_format_with_precision():
def test_format_subset():

The new module does not use `class` setup structure.

