Use a small, standalone testing framework instead of googletest.

Ninja currently uses googletest for testing. That makes building
ninja_test somewhat annoying since it requires that one passes
--with-gtest PATH to configure. It turns out just implementing the bits
of googletest that ninja uses needs about the same amount of code than
making the --with-gtest flag in configure.py work and making googletest
print test results in a way we want (!)

In addition to making configuration simpler, this also makes compiling
tests much faster: On my system, touching src/build_test.cc (the slowest
file to build in ninja) and rebuilding ninja_tests is twice as fast than
without this patch. Building all is noticeably faster too: 5.6s with
this patch, 9.1s without this patch (38% faster).

The most noticeable things missing: EXPECT_* and ASSERT_* don't support
streaming notes to them with operator<<, and for failing tests the lhs
and rhs are not printed. That's so that this header does not have to
include sstream, which slows down building ninja_test almost 20%.
If this turns out to be annoying, we can maybe add it.