[NFC][asan] Speedup uar_signals.cpp test

It was the slowest test:
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41.77s: AddressSanitizer-x86_64-linux :: TestCases/Linux/uar_signals.cpp
26.64s: AddressSanitizer-i386-linux :: TestCases/Linux/uar_signals.cpp
14.82s: AddressSanitizer-x86_64-linux :: TestCases/Posix/current_allocated_bytes.cpp
14.79s: AddressSanitizer-i386-linux :: TestCases/Posix/current_allocated_bytes.cpp
11.55s: AddressSanitizer-x86_64-linux :: TestCases/scariness_score_test.cpp
10.15s: AddressSanitizer-x86_64-linux :: TestCases/Posix/stack-use-after-return.cpp