investigate: fix flaky picker tests racing on global override

TestRunInvestigateConfigPicker_FiltersNonInstalled and TestRunInvestigateConfigPicker_QuorumExceedsAgents both installed a process-global picker-form override via SetPickerFormFnForTest while calling t.Parallel(). The atomic.Pointer prevents a data race but not the logical clobber: one test's override could replace the other's mid-run, so QuorumExceedsAgents would read the sibling's quorum=0 override and see no error — the exact flake seen in test-core. picker.go already documents that conflicting-override tests must not run in parallel. Drop t.Parallel() from both.