Fix unreliable tests

[rcoreilly]

Describe the bug

Certain tests are currently flakey. Rather than having a high probability of CI failing, masking other fixable tests, it makes more sense to skip these tests until we can fix the underlying issue.

How to reproduce

Current list of skipped tests:

• core/filepicker_test.go: TestFilePicker
• core/slider_test.go: TestSliderChange
• text/textcore/base_test.go: TestEditorInput

there are several others, all marked with this:

	t.Skip("todo: unreliable, https://github.com/cogentcore/core/issues/1641")

Example code

Relevant output

Platform

macOS