Stop ignoring whitespace while testing.

Sometime back the output of glcpp started differing from the output of
"gcc -E" in the amount of whitespace in emitted. At the time, I
switched the test suite to use "diff -w" to ignore this. This was a
mistake since it ignores whitespace entirely. (I meant to use "diff
-b" which ignores only changes in the amount of whitespace.)

So bugs have since been introduced that the test suite doesn't
notice. For example, glcpp is producing "twotokens" where it should be
producing "two tokens".

Let's stop ignoring whitespace in the test suite, which currently
introduces lots of failures---some real and some spurious.

tests/glcpp-test

     ../glcpp < $test > $test.out
     gcc -E $test -o $test.gcc
     grep -v '^#' < $test.gcc > $test.expected
-    diff -B -w -u $test.expected $test.out
+    diff -u $test.expected $test.out
 done