[gdb/testsuite] Fix gdb.base/bg-exec-sigint-bp-cond.exp for signal merging

The test-case gdb.base/bg-exec-sigint-bp-cond.exp sends 10 SIGINTS to gdb, and
counts whether 10 have arrived.

Occasionally, less than 10 arrive due to signal merging [1].

This is more likely to happen when building gdb with -fsanitize=thread.

Fix this by instead, sending one SIGINT at a time, and waiting for it to
arrive.

This still makes the test-case fail if the fix fixing the PR that the
test-case was introduced for is reverted.

Tested on aarch64-linux and x86_64-linux.

PR testsuite/32329
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32329

[1] https://www.gnu.org/software/libc/manual/html_node/Merged-Signals.html

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