simpleperf: fix test record_cmd.dump_symbols.

The test can fail when dumping an unknown symbol in a new shared
library. The shared library is recorded in DsoRecord, but the
unknown symbol may have been dumped. To fix this, don't dump
unknown symbols, and make the test not enforcing DsoRecords
and SymbolRecords to occur.

Bug: 28114205
Change-Id: Ib01d0753710c68dd9538d8b635c6d9207fe7aa1c
Test: run simpleperf_unit_test.
3 files changed