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- introduce a -k/--keep-test-files flag to prevent cleanup
- add -dstg-lint to the options that are always passed to tests
- infer library ways from the compiler to be tested instead of getting them
  from the flavour (like make)
- likewise for figuring out whether the compiler to be tested is "debugged"
- specify config.exeext
- correctly specify config.in_tree_compiler, instead of always passing True
- fix formatting of how we pass a few test options
- add (potential) extensions to check-* program names
- build check-* programs with the compiler to be tested
- set TEST_HC_OPTS_INTERACTIVE and PYTHON env vars when running tests

This reverts commit 442be690.

This is due to the failures documented in #16087. The condition here could be
improved as it matches on `BUILD_FLAVOUR` instead of looking at the compiler
flags. However, it's better than nothing and I hope we will be able to fix these
issues before long.

This was supposed to be working already but didn't work when we
specified several tests with --only. This patch not only fixes this
but also makes it possible to specify a subset of tests to run with the
TEST environment variable, like the make build system. Here are some
examples:

    hadrian/build.sh test --only=plugins01
    hadrian/build.sh test --only="plugins01 plugins02"
    TEST="plugins01 plugins02" hadrian/build.sh test
    TEST=plugins03 hadrian/build.sh test --only="plugins01 plugins02"

When both the TEST environment variable and the --only flag are used,
we simply concatenate the list of tests from both sources and ask the
testsuite driver to run them all.

This patch addresses #16026.

Test Plan: hadrian/build.sh test --only="plugins01 plugins02"

Reviewers: bgamari, snowleopard

Reviewed By: bgamari, snowleopard

Subscribers: rwbarton, carter

GHC Trac Issues: #16026

Differential Revision: https://phabricator.haskell.org/D5431