Fix "thread find" with multiple inferiors/targets (PR gdb/26631)

Fix "thread find" with multiple inferiors/targets (PR gdb/26631)

"thread find" with multiple inferiors got broken with the multi-target
work:

 Thread 1 "gdb" hit Breakpoint 1, internal_error (...) at ../../src/gdbsupport/errors.cc:51
 51      {
 (top-gdb) bt
 #0  internal_error (file=0xffffd4d0 <error: Cannot access memory at address 0xffffd4d0>, line=0, fmt=0x555556330320 "en_US.UTF-8") at ../../src/gdbsupport/errors.cc:51
 #1  0x0000555555bca4c7 in target_thread_name (info=0x555556801290) at ../../src/gdb/target.c:2035
 #2  0x0000555555beb07a in thread_find_command (arg=0x7fffffffe08e "1", from_tty=0) at ../../src/gdb/thread.c:1959
 #3  0x000055555572ec49 in do_const_cfunc (c=0x555556786bc0, args=0x7fffffffe08e "1", from_tty=0) at ../../src/gdb/cli/cli-decode.c:95
 #4  0x0000555555732abd in cmd_func (cmd=0x555556786bc0, args=0x7fffffffe08e "1", from_tty=0) at ../../src/gdb/cli/cli-decode.c:2181
 #5  0x0000555555bf1245 in execute_command (p=0x7fffffffe08e "1", from_tty=0) at ../../src/gdb/top.c:664
 #6  0x00005555559cad10 in catch_command_errors (command=0x555555bf0c31 <execute_command(char const*, int)>, arg=0x7fffffffe082 "thread find 1", from_tty=0) at ../../src/gdb/main.c:457
 #7  0x00005555559cc33d in captured_main_1 (context=0x7fffffffdb60) at ../../src/gdb/main.c:1218
 #8  0x00005555559cc571 in captured_main (data=0x7fffffffdb60) at ../../src/gdb/main.c:1243
 #9  0x00005555559cc5e8 in gdb_main (args=0x7fffffffdb60) at ../../src/gdb/main.c:1268
 #10 0x0000555555623816 in main (argc=17, argv=0x7fffffffdc78) at ../../src/gdb/gdb.c:32

The problem is that we're not switching to the inferior/target before
calling target methods, which trips on an assertion put in place
exactly to catch this sort of problem.

gdb/testsuite/ChangeLog:

	PR gdb/26631
	* gdb.multi/multi-target-thread-find.exp: New file.

gdb/ChangeLog:

	PR gdb/26631
	* thread.c (thread_find_command): Switch inferior before calling
	target methods.