tests: avoid spurious failure (line too long in awk)

tests: avoid spurious failure (line too long in awk)

* tests/parallel-tests-many.test: Use perl, not awk, to write the
Makefile.am with (deliberately) overly long TESTS content, as some
inferior awk implementations (e.g., Solaris 10 /usr/bin/awk) are
unable to handle the long lines used there and die with errors like:
"awk: string too long near line 5".