[MC] [Win64EH] Update the AArch64/seh.s test slightly. NFC.

Update the comment stating the aim of the test - this is currently
only checking that these assembler directives doesn't cause the
assembler to fail, but the results of the testcase aren't particularly
correct yet.

Remove bits of the testcase that are even less likely to be found in
the wild (the .seh_startchained/.seh_endchained block), where the
testcase currently doesn't really generate anything interesting
anyway.

Differential Revision: https://reviews.llvm.org/D86524