Add sleeps after Unity calls in testapp builder

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

The testapp builder has occasionally been having issues where after one Unity call (with -quit), when it gets to the next call Unity still seems to be running, causing an error. Adding a small sleep, to give the previous call a chance to close properly.

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Testing

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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