assert: fix wrong message indentation
If code is read from a file and that code is indented, it would be misaligned. This makes sure it has a natural indentation that is relative to the starting point of the assertion.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines