gdb/testsuite: Remove duplicates in gdb.base/catch-signal.exp

gdb/testsuite: Remove duplicates in gdb.base/catch-signal.exp

When running the testsuite I have the following:

    Running .../gdb/testsuite/gdb.base/catch-signal.exp ...
    DUPLICATE: gdb.base/catch-signal.exp: SIGHUP: continue
    DUPLICATE: gdb.base/catch-signal.exp: SIGHUP: continue
    DUPLICATE: gdb.base/catch-signal.exp: 1: continue
    DUPLICATE: gdb.base/catch-signal.exp: 1: continue
    DUPLICATE: gdb.base/catch-signal.exp: SIGHUP SIGUSR2: continue
    DUPLICATE: gdb.base/catch-signal.exp: SIGHUP SIGUSR2: continue

This patch removes DUPLICATE in gdb.base/catch-signal.exp by explicitly
giving names to the offending 'gdb_test "continue"' statements to make
them distinct.

Tested on x86_64-linux.