tests: fix under automake 1.13
tests: fix under automake 1.13

The parallel test harness (which is the default as of automake 1.13)
doesn't let you use make functions in the definition of TESTS. (It
generates an invalid Makefile in this case.) Since the tests as
currently written won't work with the parallel harness anyway (since
each apache-based test stops apache when it's done), just force the
serial harness for now.

https://bugzilla.gnome.org/show_bug.cgi?id=694135

Distrotech/libsoup,jwendell/libsoup,jwendell/libsoup,fanc999/libsoup,jwendell/libsoup,Distrotech/libsoup,fanc999/libsoup,jwendell/libsoup,Distrotech/libsoup,fanc999/libsoup,jwendell/libsoup,fanc999/libsoup,Distrotech/libsoup,Distrotech/libsoup,fanc999/libsoup