tests: Added numerous tests.

The tests are taken almost verbatim from the open source project PicoC. It can
be found at https://code.google.com/p/picoc/.

The tests range from very simple/trivial ones to more complicated. My view is
that the more tests the better. Without tests like this I was very reluctant to
make any changes to tcc for the fear of breaking things.

The tests pass on Win32, OSX, Linux x86 and x86_64. One or two tests fail on
each platform due to differences in the runtime library.

tests2/38_multiple_array_index.expect

@@ -0,0 +1,4 @@
+x=0: 1 2 3 4
+x=1: 5 6 7 8
+x=2: 9 10 11 12
+x=3: 13 14 15 16