[BEAM-13907] Improve coverage of textio package

[damccorm]

Improves the coverage of our textio package. There were a number of untested functions here and adding tests was pretty quick.

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[codecov]

Codecov Report

Merging #16937 (f78b579) into master (2a45a5b) will increase coverage by 0.44%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #16937      +/-   ##
==========================================
+ Coverage   73.00%   73.45%   +0.44%     
==========================================
  Files         658      660       +2     
  Lines       86706    86793      +87     
==========================================
+ Hits        63301    63753     +452     
+ Misses      22405    21986     -419     
- Partials     1000     1054      +54     

Flag	Coverage Δ
go	47.35% <ø> (+1.69%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdks/go/pkg/beam/util/errorx/guarded.go	100.00% <0.00%> (ø)
...s/go/pkg/beam/core/graph/window/trigger/trigger.go	59.09% <0.00%> (ø)
sdks/go/pkg/beam/io/textio/sdf.go	46.72% <0.00%> (+3.73%)	⬆️
sdks/go/pkg/beam/io/databaseio/database.go	24.24% <0.00%> (+21.96%)	⬆️
sdks/go/pkg/beam/core/runtime/exec/coder.go	56.93% <0.00%> (+26.60%)	⬆️
sdks/go/pkg/beam/util/gcsx/gcs.go	27.41% <0.00%> (+27.41%)	⬆️
sdks/go/pkg/beam/io/textio/textio.go	65.57% <0.00%> (+41.80%)	⬆️
sdks/go/pkg/beam/internal/errors/errors.go	87.50% <0.00%> (+42.70%)	⬆️
sdks/go/pkg/beam/runners/dataflow/dataflow.go	50.00% <0.00%> (+44.70%)	⬆️

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[damccorm]

R: @jrmccluskey @riteshghorse

[jrmccluskey]

LGTM mod the two nits I added.

[damccorm]

R: @lostluck

[lostluck]

LGTM, just need to wait for the tests to pass after my import removal.