Name what failed in a repository check from its whole output, not its tail (issue 460)

A long run's logs pushed the --- FAIL lines out of the last 200 lines the
summary was named from, so a failed check could not say which test failed.
Pick the lines that name a failure as the output streams, keep them whole at
the end of the verdict's report, and say the whole output in the build's log.

internal/builder/testdata/issue460/a-panic.txt

@@ -0,0 +1,18 @@
+--- FAIL: TestAMapIsNil (0.00s)
+panic: assignment to entry in nil map [recovered, repanicked]
+
+goroutine 18 [running]:
+testing.tRunner.func1.2({0x6b6810, 0x6ee000})
+	/usr/lib/go/src/testing/testing.go:2123 +0x232
+testing.tRunner.func1()
+	/usr/lib/go/src/testing/testing.go:2126 +0x329
+panic({0x6b6810?, 0x6ee000?})
+	/usr/lib/go/src/runtime/panic.go:859 +0x125
+example.com/cap/panics.TestAMapIsNil(0x1e97d59dc248?)
+	/src/cap/panics/panics_test.go:7 +0x28
+testing.tRunner(0x1e97d59dc248, 0x6d41f8)
+	/usr/lib/go/src/testing/testing.go:2193 +0xea
+created by testing.(*T).Run in goroutine 1
+	/usr/lib/go/src/testing/testing.go:2258 +0x4d4
+FAIL	example.com/cap/panics	0.004s
+FAIL