A reunion of Nick Frost and Simon Pegg—stars of the beloved comedy hybrids “Shaun of the Dead,” “Hot Fuzz,” and “The World’s End”—should produce abundant joy, especially given the promising concept of Amazon’s “Truth Seekers” to once again blend humor with another genre. And yet this 8-episode comedy is a shocking misfire, a deadly dull affair that feels much longer than even its 4-hour total runtime and produces an unbelievably small number of actual laughs and thrills.