some `isAbsolutePath()` tests fail on MinGW MSYS

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 ------ assertion failed ---------
line test.cpp:3270
expected:1
actual:0
------ assertion failed ---------
line test.cpp:3271
expected:1
actual:0
------ assertion failed ---------
line test.cpp:3276
expected:0
actual:1
------ assertion failed ---------
line test.cpp:3277
expected:0
actual:1
------ assertion failed ---------
line test.cpp:3278
expected:0
actual:1

[firewave]

The test was just using the wrong define.

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Actually MinGW MSYS appears to support Unix and Windows paths.

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Should also be tested with running a Windows executable via WSL (should probably also add that to the CI).

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This also affects Cygwin.