tests: more .NOTPARALLEL tests.

tests: more .NOTPARALLEL tests.

* contrib/t/multilib.sh: use .NOTPARALLEL to avoid unreproducible
(and undebuggable, by me) test failures when running
  make -j$njobs AM_TESTSUITE_MAKE="make -j$njobs"
for some arbitrary but rather large value of $njobs.
The failures don't occur with just make -j; have to run the makes
inside the tests in parallel also.
* t/java-compile-install.sh: likewise.
* t/java-compile-run-flat.sh: likewise.
* t/java-uninstall.sh: likewise.
* t/vala-headers.sh: likewise.