Fix GDB build in infrun.c when configured with unit tests disabled

Fix GDB build in infrun.c when configured with unit tests disabled

I noticed this while testing the GDB in the context of the upcoming
GDB 10 release branching, because part of the process involves setting
development to False, which in turn changes the default for including
unittest to false as well. As a result, without this patch, we get
compilation errors in infrun.c such as:

    infrun.c:9219:5: error: `scoped_mock_context' was not declared in this scope

This patch fixes it by bracketing the unitttest in namespace selftest
with an #if GDB_SELF_TEST.

gdb/ChangeLog:

        * infrun.c (namespace selftests): Only define #if GDB_SELF_TEST.

Tested on x86_64-linux, with and without self-tests.