tests/indent: clean up test driver

tests/indent: strengthen requirements for test input files

Previously, 'indent run-equals-prev-output' was allowed even when there
was no 'indent run' section above. This created an ambiguity, since

tests/indent: strengthen requirements for test input files

Previously, 'indent run-equals-prev-output' was allowed even when there
was no 'indent run' section above. This created an ambiguity, since
'previous output' could mean either the 'indent run' section or the
'indent run-equals-input' section.

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tests/indent: format stderr output as end-of-line comments

This generates fewer error markers in syntax-aware editors.

tests/indent: separate stdout from stderr

Due to buffering, there was no guarantee that the output would be
exactly in the same order as written in the tests.

tests/indent: skip creating an intermediate file in tests

tests/indent: fix outdated or wrong comments

tests/indent: prevent 'indent end' with arguments

tests/indent: migrate test driver from AWK to Lua

Lua reports more details when os.execute fails, which is useful when
running old versions of indent for comparison. The new test driver also
suppor

tests/indent: migrate test driver from AWK to Lua

Lua reports more details when os.execute fails, which is useful when
running old versions of indent for comparison. The new test driver also
supports multiple test files in the same run.

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