Accept duplicates when they're all the same.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>
Add instance Binary LocalBuildInfo
Use Binary instance to save/restore dist/setup-config
Load binary LocalBuildInfo in package-tests

PackageTests has its own version of getPersistBuildConfig which does not
check which version of Cabal created dist/setup-config. Now that the
LocalBuildInfo is written with Binary, this version needed to be
updated, too.
Clarify Cabal dependencies policy

[ci-skip]
Merge pull request #2076 from ttuegel/binary-lbi

Use Binary to save/restore persistent build config
Exec tests no longer fail if there isn't a sandbox

The tests which delete the sandbox, now check for its existence before
deleting it. This prevents erroneous test failures when the tests are first
run or any time that the sandbox has been deleted.
cabal-install tests use locally built cabal

When running the cabal-install tests, we want to run against the locally built
cabal executable, not any other cabal executable which may be installed.

If we are 1) following the advice of building cabal with `cabal build` (as
opposed to installing it into a sandbox with `cabal install`) and 2) not using
a non-default build directory, the locally built cabal will be located at
`dist/build/cabal/cabal`.

By adding the directory `dist/build/cabal` to the program search path we will
pick up any cabal executable there in preference to any other on the $PATH.

For most users, I expect that this will remove any surprises with an
unexpected cabal executable being selected for running the tests. It will be
sufficient to allow the tests to run on Travis.

For users who have been installing cabal into a sandbox, this change could
result in an older cabal executable being selected. It's not clear to me how
to solve this issue or whether anyone will experience this.
Enable cabal-install tests on travis
Merge pull request #2087 from benarmston/improve-cabal-install-tests

Improve cabal install tests
Support the 1.5 release of 'time'.

Fixes #2100.
Improve a debug message.
Merge pull request #2077 from ezyang/ezyang-accept-duplicates

Accept duplicates when they're all the same.
Test executable tests without hpc
Add printRawCommandAndArgsAndEnv
Enable hpc with LocalBuildInfo flag
Enable program coverage for all components
Clean up after HPC tests properly
Useful error message for --enable-library-coverage
Overhaul HPC support

This fixes issue #1945 and replaces pull request #1947.

These bugs in #1947 are fixed:
* The pull request no longer applied cleanly to master because of the
  Binary LocalBuildInfo patches.
* The new test cases did not put the generated HPC interface files under
  "dist/", so the files weren't cleanup up correctly.
* HPC interface files for each component need to be separated, but the
  were being combined into the same directory.

These enhancements are also implemented:
* Program coverage is now used for all components when enabled and the
  flag --enable-library-coverage has been changed to --enable-coverage.
* A helpful error message is printed if the obsolete flag
  --enable-library-coverage is used.