test: use realpath for NODE_TEST_DIR in common.js

[gibfahn]

If you don't specify NODE_TEST_DIR, common.tmpDir will resolve to the
realpath of node/test.

If you do specify NODE_TEST_DIR (with the environment variable or by
running tools/test.py --test-dir=x), common.tmpDir (which
is resolved from testRoot) uses the symbolic path (doesn't resolve
symlinks). This uses fs.realpathSync() to fix that.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

test, common