GSettings tests: reverse installed test complexity

GSettings tests: reverse installed test complexity

Remove the complications that were introduced in an attempt to make the
gsettings and gschema-compile tests function as installed tests.  These
tests are designed (in large part for gsettings and entirely for
gschema-compile) to test the in-tree tools and should not be testing the
system versions.

In the future we may want to move the use of the in-tree tools from the
gsettings testcase into the Makefile and install the resulting files,
allowing this testcase to run against those files, installed.