[gdb/testsuite] Fix gdb.base/infcall-exec.exp for remote target

[gdb/testsuite] Fix gdb.base/infcall-exec.exp for remote target

When running test-case gdb.base/infcall-exec.exp with target board
remote-gdbserver-on-localhost (using REMOTE_TARGET_USERNAME) we run into:
...
(gdb) call (int) execlp ("$outputs/gdb.base/infcall-exec/infcall-exec2", \
  "$outputs/gdb.base/infcall-exec/infcall-exec2", (char *)0)^M
$1 = -1^M
(gdb) FAIL: gdb.base/infcall-exec.exp: call execlp
...

Fix this by using just:
...
(gdb) call (int) execlp ("infcall-exec2", "infcall-exec2", (char *)0)^M
...
and using putenv ("PATH=...") to allow infcall-exec to exec infcall-exec2
if it's available alongside.

Also fix the exec name in the test-case, such that we can successfully
run the test-case:
...
$ ./outputs/gdb.base/infcall-exec/infcall-exec
PATH SETTING: 'PATH=./outputs/gdb.base/infcall-exec'
$
...

Tested on x86_64-linux.

Co-Authored-by: Ivan Tetyushkin <ivan.tetyushkin@syntacore.com>