rt-tests: fix mips+uclibc check

The conditional (suggested by me) introduced in 108952 (rt-tests: disable
for uclibc mips) isn't actually valid kconfig syntax, causing menuconfig to
error out.

Rewrite it to use proper syntax.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
rt-tests: cleaning up
Merge remote-tracking branch 'mirror/master'