selftest: Taint kernel when test module loaded

Make any kselftest test module (using the kselftest_module framework)
taint the kernel with TAINT_TEST on module load.

Also mark the module as a test module using MODULE_INFO(test, "Y") so
that other tools can tell this is a test module. We can't rely solely
on this, though, as these test modules are also often built-in.

Finally, update the kselftest documentation to mention that the kernel
should be tainted, and how to do so manually (as below).

Note that several selftests use kernel modules which are not based on
the kselftest_module framework, and so will not automatically taint the
kernel.

This can be done in two ways:
- Moving the module to the tools/testing directory. All modules under
this directory will taint the kernel.
- Adding the 'test' module property with:
MODULE_INFO(test, "Y")

Similarly, selftests which do not load modules into the kernel generally
should not taint the kernel (or possibly should only do so on failure),
as it's assumed that testing from user-space should be safe. Regardless,
they can write to /proc/sys/kernel/tainted if required.

Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Acked-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

kselftest: add support for skipped tests

Update the kselftest framework to allow client drivers to
specify that some tests were skipped.

Signed-off-by: Timur Tabi <timur@kernel.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210214161348.369023-3-timur@kernel.org

kselftest: Add test module framework header

kselftest runs as a userspace process. Sometimes we need to test things
from kernel space. One way of doing this is by creating a test module.
Currently doing so requires developers to write a bunch of boiler plate
in the module if kselftest is to be used to run the tests. This means
we currently have a load of duplicate code to achieve these ends. If we
have a uniform method for implementing test modules then we can reduce
code duplication, ensure uniformity in the test framework, ease code
maintenance, and reduce the work required to create tests. This all
helps to encourage developers to write and run tests.

Add a C header file that can be included in test modules. This provides
a single point for common test functions/macros. Implement a few macros
that make up the start of the test framework.

Add documentation for new kselftest header to kselftest documentation.

Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tobin C. Harding <tobin@kernel.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>