internal/rosa/llvm: enable LLVM_BUILD_TESTS

This arranges for tests to be built early, and is more efficient towards the end of the build.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/llvm.go

@@ -251,6 +251,7 @@ func (t Toolchain) newLLVM() (pkg.Artifact, string) {
 			// symbols: clock_gettime, mallopt
 			{"COMPILER_RT_INCLUDE_TESTS", "OFF"},
 
+			{"LLVM_BUILD_TESTS", "ON"},
 			{"LLVM_LIT_ARGS", litArgs(true, skipChecks...)},
 		}...)
 	}