tests: rename default plan to build-gating to better eflect its current purpose

My expectation is that in the future, more tests will land in this git,
and not all of them would be useful in gating time. AFAIK, CI systems do
run everything they can, therefore adding `when` to control when the
plan is executed - `is not defined` bit should allow anyone to run it
manually, while the rest of the condition should limit its presence to
gating events.

tests/build-gating.fmf

@@ -0,0 +1,19 @@
+summary: LLVM tests for build/PR gating
+adjust:
+  - because: "Plan to be ran when either executed locally, or executed by CI system to gate a build or PR."
+    when: >-
+      trigger is defined
+      and trigger != commit
+      and trigger != build
+    enabled: false
+discover:
+    how: fmf
+execute:
+    how: tmt
+prepare:
+    - name: packages
+      how: install
+      package: llvm
+provision:
+  hardware:
+    memory: ">= 4 GiB"

tests/default.fmf

@@ -1,12 +0,0 @@
-summary: All available LLVM tests
-discover:
-    how: fmf
-execute:
-    how: tmt
-prepare:
-    - name: packages
-      how: install
-      package: llvm
-provision:
-  hardware:
-    memory: ">= 4 GiB"