Few questions about semmle-extractor-options

Hey team!

1. Is there any documentation about `semmle-extractor-options`? I see some docs that mention extractor options but I can't find a list of available extractor options (like `--expect_errors`, etc). I usually look for how `semmle-extractor-options` is used in the existing CodeQL tests but that's not very convenient and I can probably miss some useful options.

2. I see `${testdir}` in some `semmle-extractor-options`. Is it possible to set custom variables? I tired something like `export myvar=... ; codeql test run ...` but it throws `Attempting to expand unknown variable ...` error.

The issue concerns documentation for semmle-extractor-options, including available options and custom variable expansion, and points to existing CodeQL tests as current references. Start by reviewing those test usages and the extractor-options documentation mentioned in the issue. Done means the supported options and variable behavior are documented clearly.

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