[gdb/testsuite] Use valnum_re more often (part 3)

Also transform '\\\$\[0-9\].*' into valnum_re, assuming that '\\\$\[0-9\]+' was
meant.

Done using these commands to cover '\\\$' vs '\\$' and '\[0-9\]' vs '\[0-9]':
...
find gdb/testsuite/ -type f -name *.exp* \
    | xargs sed -i 's/\\\\\\$\\\[0-9\\\]\.\*/${::valnum_re}/g'
find gdb/testsuite/ -type f -name *.exp* \
    | xargs sed -i 's/\\\\\\$\\\[0-9\]\.\*/${::valnum_re}/g'
find gdb/testsuite/ -type f -name *.exp* \
    | xargs sed -i 's/\\\\$\\\[0-9\\\]\.\*/${::valnum_re}/g'
find gdb/testsuite/ -type f -name *.exp* \
    | xargs sed -i 's/\\\\$\\\[0-9\]\.\*/${::valnum_re}/g'
...

Approved-by: Kevin Buettner <kevinb@redhat.com>