Short-circuit check --rerun if testdir doesn't exist

[muzimuzhi]

When corresponding testdir doesn't exist, currently l3build check --rerun exits with all testdirs created as files in builddir, not as directories.

Unfortunately a following l3build check without --rerun won't reset this wrong state, because none of mkdir() and cleandir() ensures the the created/cleaned object is a directory. Thus user has to clean builddir manually to recover from the problem.

This PR adds a guard to check() so that l3build check --rerun will complain and short-circuit when the corresponding testdir doesn't exist.

• Before

  $ git clone [email protected]:latex3/l3build.git && cd l3build
  # specify a config file to get shorter stdout/stderr
  $ texlua ./l3build check -c build --rerun
  Running checks on
    00-test-1 (1/1)
  ./l3build-file-functions.lua:184: Unable to change working directory to './build/test'
  Not a directory
  $ tree build
  build
  └── test
  
  1 directory, 1 file

  Note the ./build/test is created as a file, not a directory.
• After

  $ texlua ./l3build.lua check --rerun -c build
  
    Test directory "./build/test" doesn't exist. Try again without "--rerun".
  $ tree build
  build  [error opening dir]
  
  0 directories, 0 files

  Without -c build

  $ texlua ./l3build.lua check --rerun
  Running l3build with target "check" and configuration "build"
  
    Test directory "./build/test" doesn't exist. Try again without "--rerun".
  
  Running l3build with target "check" and configuration "config-pdf"
  
    Test directory "./build/test-config-pdf" doesn't exist. Try again without "--rerun".
  
  Running l3build with target "check" and configuration "config-plain"
  
    Test directory "./build/test-config-plain" doesn't exist. Try again without "--rerun".
  
  Running l3build with target "check" and configuration "config-context"
  
    Test directory "./build/test-config-context" doesn't exist. Try again without "--rerun".
  
  Failed tests for configuration "build":
  
    Check failed with difference files
  
  ...
  
  $ tree build
  build  [error opening dir]
  
  0 directories, 0 files