[docs] GettingStarted.md: Do not suggest to run tests when building toolchain
[docs] GettingStarted.md: Do not suggest to run tests when building toolchain

* Running the entire test suite is unnecessarily burdensome for newcomers, given
  the already considerable build times.
* Test failures are not a beginner-friendly indication of an out-of-sync
  checkout, nor is such an indication generally worth the expenses.
* We do not want a known issue to block anyone’s workflow.

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