Create compat18 branch

tests/basic/main.fmf

@@ -7,7 +7,7 @@ extra-nitrate: TC#0614058
 adjust+:
   # Common requirements when LLVM is not SCL-ized
   - require+:
-      - clang
+      - clang18
     when: collection is not defined
 
   # Requirements for SCL-ized LLVM

tests/ld-alternative/main.fmf

@@ -13,3 +13,7 @@ adjust+:
   - because: lld is not installed as ld alternative in rhel < 9
     enabled: false
     when: distro < rhel-9
+
+  - because: lld not installed as alternative in compat packages
+    enabled: false
+    when: compat is defined

tests/main.fmf

@@ -14,7 +14,7 @@ framework: shell
 
 # Always define "test" key and override in adjust or in each specific test
 # metadata if needed.
-test: ./test.sh
+test: "PATH=/usr/lib64/llvm18/bin:$PATH ./test.sh"
 
 # Commonly used keys are initialized to empty.
 require: []
@@ -34,7 +34,7 @@ adjust+:
 
   # Common requirements when LLVM is not SCL-ized
   - require+:
-      - lld
+      - lld18
     when: collection is not defined
 
   # Requirements for SCL-ized LLVM