[gdb/testsuite] Fix gdb.base/trace-commands.exp with editing off

With test-case gdb.base/trace-commands.exp and editing off, I run into fails
because multi-line commands are issued using gdb_test_sequence, which
doesn't handle them correctly.

Fix this by using gdb_test instead.

Tested on x86_64-linux.

PR testsuite/30288
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30288