gh-113903: Fix an IDLE configdialog test (#113973)

test_configdialog.HighPageTest.test_highlight_target_text_mouse fails
if a line of the Highlight tab text sample is not visible. If so, bbox()
in click_char() returns None and the unpacking iteration fails.

This occurred on a Devuan Linux system. Fix by moving the
'see character' call inside click_char, just before the bbox call.

Also, reduce the click_char calls to just one per tag name and
replace the other nested function with a dict comprehension.