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-data-race.txt

@@ -0,0 +1,31 @@
+==================
+WARNING: DATA RACE
+Read at 0x00c000018398 by goroutine 8:
+  example.com/cap/race.TestACounterIsShared.func1()
+      /src/cap/race/race_test.go:13 +0x7b
+
+Previous write at 0x00c000018398 by goroutine 9:
+  example.com/cap/race.TestACounterIsShared.func1()
+      /src/cap/race/race_test.go:13 +0x8d
+
+Goroutine 8 (running) created at:
+  example.com/cap/race.TestACounterIsShared()
+      /src/cap/race/race_test.go:13 +0x78
+  testing.tRunner()
+      /usr/lib/go/src/testing/testing.go:2193 +0x21c
+  testing.(*T).Run.gowrap1()
+      /usr/lib/go/src/testing/testing.go:2258 +0x38
+
+Goroutine 9 (finished) created at:
+  example.com/cap/race.TestACounterIsShared()
+      /src/cap/race/race_test.go:13 +0x78
+  testing.tRunner()
+      /usr/lib/go/src/testing/testing.go:2193 +0x21c
+  testing.(*T).Run.gowrap1()
+      /usr/lib/go/src/testing/testing.go:2258 +0x38
+==================
+--- FAIL: TestACounterIsShared (0.00s)
+    testing.go:1865: race detected during execution of test
+FAIL
+FAIL	example.com/cap/race	0.008s
+FAIL