Fix the directory calculation for when using paratesting.

With #647 I moved this block out of the if ($examples == 1) { block, but the
parnodefile piece should only be run inside the the block.

The concatenation of testdirs to testflags needs to happen outside, in case
CHPL_NIGHTLY_TEST_DIRS is set (and no flags are given).
Fix not equal string operator.
Merge pull request #654 from thomasvandoren/fix-valgrind-nightly-testing

Fix valgrind nightly testing.

I moved a block into the global scope in #647 that caused a DIRFILE to be created, which included directories such as ./Share that are not intended to be run. Move that part of the block back to its original location, leaving the half that concats testdirs onto testflags when not using paratest.