test/goroot: fix bug449 runoutput handling

[cpunion]

Summary

• generate runoutput temporary source once with baseline Go, then build/run that generated file with Go and LLGo
• count generated-source compilation against build timeout instead of run timeout
• add runner coverage proving LLGo is not used to run the runoutput generator
• remove fixed bug449 runoutput xfail/timeout entries

Local verification

• go test ./test/goroot -run 'TestRunOutputCaseGeneratesWithBaselineGoOnly|TestRunGeneratedProgramUsesProvidedTimeout|TestEnsureModuleWorkspace|TestSplitSourceFiles' -count=1 -v\n- go test ./test/goroot -count=1\n- go test ./test/goroot -run TestGoRootRunCases -count=1 -timeout 20m -v -args -goroot "$(go env GOROOT)" -dirs fixedbugs -case '^fixedbugs/bug449\.go$' -directive-mode ci -xfail /tmp/llgo-goroot-empty-xfail.yaml -progress 30s\n- bash dev/test_goroot.sh /Users/lijie/sdk/go1.25.0 -- -dirs fixedbugs -case '^fixedbugs/bug449\.go$' -directive-mode ci -xfail /tmp/llgo-goroot-empty-xfail.yaml -progress 30s\n- bash dev/test_goroot.sh /Users/lijie/sdk/go1.26.0 -- -dirs fixedbugs -case '^fixedbugs/bug449\.go$' -directive-mode ci -xfail /tmp/llgo-goroot-empty-xfail.yaml -progress 30s\n- go test ./test/goroot -run TestGoRootRunCases -count=1 -timeout 20m -v -args -goroot "$(go env GOROOT)" -dirs fixedbugs -case '^fixedbugs/bug449\.go$' -directive-mode ci -progress 30s

[codecov-commenter]

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