tools/mixin/bin/test-all - Fix guard for EXT_DIR

[totten]

Overview

This patches a test/infra script and does not affect runtime functionality.

Follow-up to #22198. CC @seamuslee001

Before

tools/mixin/bin/test-all only runs if there is a pre-existing copy of the example extension ($extensionsDir/example-mixin). If you attempt to run on a clean build (where there is no example extension yet), then it complains that the directory does not exist.

After

tools/mixin/bin/test-all only asserts that the $extensionsDir exists. It auto-creates $extensionsDir/example-mixin as needed.

[civibot]

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[totten]

Test failure is unrelated.