cabal-testsuite #8401: communicate better that --with-cabal is needed

- Add caveat to `--help` text.
- Put `--with-cabal` into very first example in README.

cabal-testsuite/README.md

@@ -7,8 +7,8 @@ How to run
 1. Build `cabal-testsuite` (`cabal build cabal-testsuite:cabal-tests`)
 2. Run the `cabal-tests` executable. It will scan for all tests
    in your current directory and subdirectories and run them.
-   To run a specific set of tests, use `cabal-tests PATH ...`.
-   (e.g. `cabal run cabal-testsuite:cabal-tests -- cabal-testsuite/PackageTests/TestOptions/setup.test.hs`)
+   To run a specific set of tests, use `cabal-tests --with-cabal=CABALBIN PATH ...`.
+   (e.g. `cabal run cabal-testsuite:cabal-tests -- --with-cabal=cabal cabal-testsuite/PackageTests/TestOptions/setup.test.hs`)
    You can control parallelism using the `-j` flag.
 
 There are a few useful flags:

cabal-testsuite/src/Test/Cabal/Monad.hs

@@ -104,7 +104,7 @@ data CommonArgs = CommonArgs {
 commonArgParser :: Parser CommonArgs
 commonArgParser = CommonArgs
     <$> optional (option str
-        ( help "Path to cabal-install executable to test"
+        ( help "Path to cabal-install executable to test. If omitted, tests involving cabal-install are skipped!"
        <> long "with-cabal"
        <> metavar "PATH"
         ))