Tests: Refactor key modifier checking.

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Test: Add Shift-Enter to switch between FS and desktop FS.
Allow switching between FS and desktop FS.

This should fix bug #2057 (https://bugzilla.libsdl.org/show_bug.cgi?id=2057)

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Mac: Don't give windows focus back when we're closing them.

Fixes bug #1840 (https://bugzilla.libsdl.org/show_bug.cgi?id=1840)

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Don't minimize fullscreen windows when destroying them.

Previously, we'd minimize fullscreen windows (if
SDL_HINT_VIDEO_MINIMIZE_ON_FOCUS_LOSS was set) during SDL_DestroyWindow if they
had keyboard focus, because we call SDL_SetKeyboardFocus(NULL) which yields a
OnWindowsFocusLost event.

Related to https://bugzilla.libsdl.org/show_bug.cgi?id=1840

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