gdb/testsuite: Reduce indentation in gdb.threads/threadcrash.exp

In test-case gdb.threads/threadcrash.exp we have an unnecessarily indented
gdb_test_multiple:
...
    gdb_test_multiple "thread apply all backtrace" \
	"Get thread information" -lbl {
	    -re "#\[0-9\]+\\\?\\\?\[^\n\]*" {
...

Fix this by moving the command into a variable, allowing the
"gdb_test_multiple ... {" to fit on a single 80 chars line.

Tested on arm-linux and x86_64-linux.