[gdb/testsuite] Fix filename in gdb.debuginfod/crc_mismatch.exp

[gdb/testsuite] Fix filename in gdb.debuginfod/crc_mismatch.exp

After running test-case gdb.debuginfod/crc_mismatch.exp, I find a dir called '$':
...
$ ls $build/gdb/testsuite/
$      config.log     gdb.log  lib       outputs   site.exp
cache  config.status  gdb.sum  Makefile  site.bak  temp
...

Fix this by removing the stray '$' here:
...
set debugfile "$[standard_output_file ${testfile}.debug]"
...

Tested on x86_64-linux.