ImathTest fails on mingw32

ImathTest fails with what appear to be numerical precision issues:

```
Assertion failed: f1.worldRadius (v3, one) == f1.worldRadiusExc (v3, one), file D:/a/Imath/Imath/src/ImathTest/testFrustum.cpp, line 449
Assertion failed: b21 == b2, file D:/a/Imath/Imath/src/ImathTest/testBoxAlgo.cpp, line 830

```

To activate the failure in the CI checks, remove this line in `.githhub/work/ci_workflow.yml`:

` BUILD_TESTING: 'OFF' # TODO: Imath.testMatrix fails, please help find why!
`

Start with the failing assertions in Imath/src/ImathTest/testFrustum.cpp at line 449 and Imath/src/ImathTest/testBoxAlgo.cpp at line 830. Re-enable BUILD_TESTING in .githhub/work/ci_workflow.yml and reproduce the failures on mingw32. Done means ImathTest passes there without the reported precision assertions.

Written by the indexing model from the issue text.