test_sparse_array: don't put main() in a namespace, fix a linking error

The main() function is expected to be a global function, the code won't
link as a standalone executable if main() is put into a namespace.

This fixes the following linking error:

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[ 32%] Linking CXX executable ../../../bin/test_sparse_array
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/Scrt1.o: in function `_start':
(.text+0x20): undefined reference to `main'
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