testsuite: Be explicit about runtime test dependencies

Issue haskell#8356 reports occasional errors from running the testsuite about
multiple package versions available. This stems from the invokation of
`runghc` not being explicit about all dependencies of the testsuite.

The solution is provide a component in the cabal file which is explicit
about which packages the tests can depend on. This component has a
build-depends section which lists all the dependencies that the tests
require.

It would be better if this component was a library component but we
can't do this with a Custom setup because of limitations to do with
per-component builds.

Then we also enable `-hide-all-packages`, so the dependency will not be
available if it is not explicitly listed as a dependency.

You could also imagine a future where the Setup.hs script found the test
files and compiled a single executable which would run all the tests,
rather than invoking runghc on each one individually.

Fixes haskell#8356

changelog.d/issue-6006

@@ -0,0 +1,15 @@
+synopsis: Fix assertion failure when --enable-documentation is enabled
+packages: cabal-install
+prs: #9446
+issuesa: #6006
+
+description: {
+
+If any non-library components ended up in the install plan for a library and
+--enable-documentation was enabled, then cabal would throw an assertion failure
+due to mismatched hashes. Now the relevant part of the package description is not
+updated after the hash has been calculated, and the check about whether to actually build documentation happens
+at the point documentation is built.
+
+
+}