Parser for Oils spec test format (.test.sh files).

A file is a run of cases. Each case opens with #### <name>; every line after that up to the next #### is either a ## directive or a line of the script, in any order — Oils puts ## SKIP above the script and the expectations below it.

Directives:

• ## stdout: LINE / ## stderr: LINE — one expected line. The value may be empty (## stdout: with nothing after the colon), which is how the format spells "one empty line".
• ## stdout-json: "…" / ## stderr-json: "…" — an escaped expectation, the only way to express output with no trailing newline
• ## STDOUT: … ## END / ## STDERR: … ## END — multiline
• ## status: N — expected exit status (default 0)
• ## SKIP (why): reason — do not run this case
• ## N-I <shells> KEY: …, ## BUG <shells> KEY: …, ## OK <shells> KEY: … (and the -2/-3 numbered spellings) — what those shells actually do, overriding the unannotated default for them per key. The unannotated default records osh, so an annotation naming bash is bash's recorded behaviour and is the expectation we want; just-bash is this repo's own recorded behaviour and wins over bash. An annotation naming neither is dropped, and a multiline one is consumed to ## END so its body does not fall through into the script.
• ## compare_shells:, ## oils_failures_allowed:, ## tags:, … — file- or case-level metadata we have no use for

The script is kept byte-for-byte between its first and last non-blank line. Only whole blank lines at either end — the file's layout, not the case's — are dropped: $LINENO, set -x traces and bash's own line N diagnostics count from the first surviving line, and a trailing space on the last command is part of what the case tests.

Example:

#### Add one to var
i=1
echo $(($i+1))
## stdout: 2