Extended tests to work with post-synth RTL

[nandor]

Includes improvements to post-synth simulations:

• Added a QUICK strategy which tries to get Vivado to run fast, yet still helps us reproduce failures
• Limited Vivado to 1 thread to mitigate flakyness from parallel synthesis
• The harness now explicitly waits for GSR
• Integrated post-synth metasims with the harness. Setting TEST_DISABLE_VIVADO=1 in the environment disables these tests even if Vivado is available.

Related PRs / Issues

UI / API Impact

Verilog / AGFI Compatibility

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[fabianschuiki]

LGTM!