Restored testing of ImageFont class

[radarhere]

#7244 added the following test code

Pillow/Tests/test_imagefont.py

Lines 1056 to 1060 in 78b96c0

	default_font = ImageFont.load_default()
	with pytest.raises(ValueError):
	default_font.getlength("A" * 1_000_001)
	with pytest.raises(ValueError):
	default_font.getbbox("A" * 1_000_001)

At the time, this was testing the "better than nothing" font that uses ImageFont class. However, #7354 changed the default font in some environments, so the ImageFont class would no longer have been covered in this test.

This PR fixes that by creating an instance of the ImageFont class directly.

[nulano]

I thought using a not-fully-initialized instance of ImageFont would not be a good test, but adding ImageFont.MAX_STRING_LENGTH = None, I see that this fails with AttributeError instead, confirming this fixes the test.

As far as testing the rest of ImageFont (i.e. coverage after the string length check), I have since found test_font_pcf.py uses an instance initialized with ImageFont.load.