[gdb/testsuite] Set up dap log file in gdb.dap/type_checker.exp

[gdb/testsuite] Set up dap log file in gdb.dap/type_checker.exp

While debugging a slow-down in test-case gdb.dap/type_checker.exp due to a WIP
patch, I noticed that test-case gdb.dap/type_checker.exp doesn't have a dap
log file.

This is normally set up by dap_gdb_start, but the test-case doesn't use it.

Fix this by doing "set debug dap-log-file $logfile" in the test-case.

To make it easy to do so, I've factored out a new proc new_dap_log_file, and
while at likewise a new proc current_dap_log_file.

Note that the log file is currently empty.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>