[gdb/testsuite] Reimplement gdb.gdb/python-interrupts.exp as unittest

[gdb/testsuite] Reimplement gdb.gdb/python-interrupts.exp as unittest

The test-case gdb.gdb/python-interrupts.exp:
- runs to captured_command_loop
- sets a breakpoint at set_active_ext_lang
- calls a python command
- verifies the command triggers the breakpoint
- sends a signal and verifies the result

The test-case is fragile, because (f.i. with -flto) it cannot be guaranteed
that captured_command_loop and set_active_ext_lang are available for setting
breakpoints.

Reimplement the test-case as unittest, using:
- execute_command_to_string to capture the output
- try/catch to catch the "Error while executing Python code" exception
- a new hook selftests::hook_set_active_ext_lang to raise the signal

Tested on x86_64-linux.