Reporter: test truncated .coverage files

Part of #385.

test/report/truncate.t

@@ -0,0 +1,149 @@
+A successful run with a complete .coverage file.
+
+  $ echo "(lang dune 2.7)" > dune-project
+  $ cat > dune <<'EOF'
+  > (executable
+  >  (name test)
+  >  (instrumentation (backend bisect_ppx)))
+  > EOF
+  $ dune exec ./test.exe --instrument-with bisect_ppx
+  $ mv bisect*.coverage bisect0.coverage
+  $ cat bisect0.coverage
+  BISECT-COVERAGE-4 1 7 test.ml 6 13 30 45 245 229 212 6 1 0 1 0 0 0
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Coverage: 2/6 (33.33%)
+
+
+Truncate the .coverage file, clipping one of the integer arrays.
+
+  $ truncate -c -s 56 bisect0.coverage
+  $ cat bisect0.coverage
+  BISECT-COVERAGE-4 1 7 test.ml 6 13 30 45 245 229 212 6 1
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Error: cannot read coverage file 'bisect0.coverage': bad integer
+  [1]
+
+  $ truncate -c -s 55 bisect0.coverage
+  $ cat bisect0.coverage
+  BISECT-COVERAGE-4 1 7 test.ml 6 13 30 45 245 229 212 6 
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Error: cannot read coverage file 'bisect0.coverage': bad integer
+  [1]
+
+  $ truncate -c -s 54 bisect0.coverage
+  $ cat bisect0.coverage
+  BISECT-COVERAGE-4 1 7 test.ml 6 13 30 45 245 229 212 6
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Error: cannot read coverage file 'bisect0.coverage': bad integer
+  [1]
+
+
+Truncate the whole array.
+
+  $ truncate -c -s 53 bisect0.coverage
+  $ cat bisect0.coverage
+  BISECT-COVERAGE-4 1 7 test.ml 6 13 30 45 245 229 212 
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Error: cannot read coverage file 'bisect0.coverage': bad integer
+  [1]
+
+  $ truncate -c -s 52 bisect0.coverage
+  $ cat bisect0.coverage
+  BISECT-COVERAGE-4 1 7 test.ml 6 13 30 45 245 229 212
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Error: cannot read coverage file 'bisect0.coverage': bad integer
+  [1]
+
+
+Truncate a string.
+
+  $ truncate -c -s 30 bisect0.coverage
+  $ cat bisect0.coverage
+  BISECT-COVERAGE-4 1 7 test.ml 
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Error: cannot read coverage file 'bisect0.coverage': bad integer
+  [1]
+
+  $ truncate -c -s 29 bisect0.coverage
+  $ cat bisect0.coverage
+  BISECT-COVERAGE-4 1 7 test.ml
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Error: cannot read coverage file 'bisect0.coverage': bad integer
+  [1]
+
+  $ truncate -c -s 28 bisect0.coverage
+  $ cat bisect0.coverage
+  BISECT-COVERAGE-4 1 7 test.m
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Error: coverage file 'bisect0.coverage' is truncated
+  [1]
+
+  $ truncate -c -s 22 bisect0.coverage
+  $ cat bisect0.coverage
+  BISECT-COVERAGE-4 1 7 
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Error: coverage file 'bisect0.coverage' is truncated
+  [1]
+
+  $ truncate -c -s 21 bisect0.coverage
+  $ cat bisect0.coverage
+  BISECT-COVERAGE-4 1 7
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Error: coverage file 'bisect0.coverage' is truncated
+  [1]
+
+  $ truncate -c -s 20 bisect0.coverage
+  $ cat bisect0.coverage
+  BISECT-COVERAGE-4 1 
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Error: cannot read coverage file 'bisect0.coverage': bad integer
+  [1]
+
+
+Truncate the file header.
+
+  $ truncate -c -s 18 bisect0.coverage
+  $ cat bisect0.coverage
+  BISECT-COVERAGE-4 
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Error: cannot read coverage file 'bisect0.coverage': bad integer
+  [1]
+
+  $ truncate -c -s 17 bisect0.coverage
+  $ cat bisect0.coverage
+  BISECT-COVERAGE-4
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Error: cannot read coverage file 'bisect0.coverage': bad integer
+  [1]
+
+  $ truncate -c -s 16 bisect0.coverage
+  $ cat bisect0.coverage
+  BISECT-COVERAGE-
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Error: coverage file 'bisect0.coverage' is truncated
+  [1]
+
+
+Truncate the whole file.
+
+  $ truncate -c -s 0 bisect0.coverage
+  $ cat bisect0.coverage
+  $ bisect-ppx-report summary --verbose
+  Info: found *.coverage files in './'
+  Error: coverage file 'bisect0.coverage' is truncated
+  [1]