math: Rewrite Vec tests and fix Vec cross calculation bug

[noisegul]

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Rewrote the old commented out tests in accordance to the Mach style guide and added the listed missing basic tests.
While doing so I also caught two bugs in the cross and swizzling methods:

• the naming of the Vec3 swizzling method was wrong, using yzw instead of yzx, and .w() instead of .x(), while cross was calling yzx
• the cross method used e.g. .mul(b.xzy()) instead of .mul(&b.xzy())

[slimsag]

This is super, super helpful - thank you!