[Ada] Fix number of lines in -ada-task-info output

[Ada] Fix number of lines in -ada-task-info output

When using the new -ada-task-info command with an argument,
the output would say that there are N entries in the returned
table, (where N is the total number of tasks present in the inferior).
But, in fact, the table would only contain at most 1 entry.

This patch fixes this by properly computing the number of
tasks being displayed before giving it to the uiout.

gdb/ChangeLog:

        * ada-tasks.c (print_ada_task_info): Fix computation of
        number of tasks displayed in command output.

gdb/testsuite/ChangeLog:

        * gdb.ada/mi_task_info/task_switch.adb: New file.
        * gdb.ada/mi_task_info.exp: New file.