move helper gating script to tests dir

Sometimes changes to the helper script for the gating plan are needed
without a ticket. Since this is a shell script, it is not exempt from
dist-git policy that requires a ticket for any non-test change. The
gating plan itself is a fmf file which is exempt, so only this used to
be a problem in the past.

Because of the STI test format, the whole tests folder is exempt too.
Moving the helper script there to avoid the need for tickets.

plan-gating.fmf

@@ -8,21 +8,21 @@ discover:
     tests:
         - name: tier0
           test: |
-              ./prepare-tests.sh
+              ./tests/prepare-tests.sh
               $TMT_SOURCE_DIR/pcs-*/pcs_test/suite --tier0 -v --vanilla --installed
           duration: 10m
           tier: 0
           tag: fast
         - name: tier1
           test: |
-              ./prepare-tests.sh
+              ./tests/prepare-tests.sh
               $TMT_SOURCE_DIR/pcs-*/pcs_test/suite --tier1 -v --vanilla --installed
           duration: 2h
           tier: 1
         - name: smoke
           test: |
               systemctl start pcsd
-              ./prepare-tests.sh
+              ./tests/prepare-tests.sh
               $TMT_SOURCE_DIR/pcs-*/pcs_test/smoke.sh
           duration: 10m
           tier: smoke