Modify some tests for absolute trees containing whitespace. * tests/missing.test: Use "./missing" instead of "`pwd`/missing". * tests/missing2.test: Likewise. * tests/mmodely.test: Add `pwd` to $PATH instead of absolute references to myyacc.sh and mylex.sh. * tests/multlib.test: Likewise for mycc. * tests/txinfo26.test: Skip if `pwd` contains whitespace, as an absolute --srcdir does not work there.