eclass/tests/toolchain-funcs.sh add test for 'tc-cpp-is-true()'

eclass/tests/toolchain-funcs.sh add test for 'tc-cpp-is-true()'

With patch from bug #698912 reverted we observe failures as:
 * Testing tc-cpp-is-true (gcc, defined) ...              [ ok ]
 * Testing tc-cpp-is-true (gcc, not defined) ...          [ ok ]
 * Testing tc-cpp-is-true (gcc, defined on -ggdb3) ...    [ !! ]
 * Testing tc-cpp-is-true (clang, defined) ...            [ !! ]
 * Testing tc-cpp-is-true (clang, not defined) ...        [ ok ]
 * Testing tc-cpp-is-true (clang, defined on -ggdb3) ...  [ !! ]

Closes: https://bugs.gentoo.org/698912
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>