TOOLS-4278 Remove the last GoConvey from mongoimport

[autarch]

Converts TestHiddenOptionsDefaults and TestImportMIOSOE, the last two
Convey/So sites in the package, and drops the goconvey dot-import.
mongoimport is now entirely testify.

TestImportMIOSOE asserted ShouldNotBeNil twice on the same unmodified
err, so those collapse into one require.Error. Its two ShouldAlmostEqual
calls become InDelta, with the float64 on the delta as that signature
requires.

Tasks earlier in this stack each extracted a fixture-opening helper into
the file they were converting, and the three came out identical apart
from a few words of doc comment. They collapse here into one
openTestFixture in a new helpers_test.go, now that the whole package is
converted and the duplication is visible.

No behavior change.

[autarch]

Warning

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• TOOLS-4278 Clean up after the testify migration #1090
• TOOLS-4278 Drop the goconvey dependency #1089
• TOOLS-4278 Remove the last GoConvey from the repo #1088
• TOOLS-4278 Convert mongodump oplog, query and view tests to testify #1087
• TOOLS-4278 Convert mongodump BSON and metadata tests to testify #1086
• TOOLS-4278 Remove the last GoConvey from mongofiles #1085
• TOOLS-4278 Convert mongofiles write concern tests to testify #1084
• TOOLS-4278 Remove the last GoConvey from mongorestore #1083
• TOOLS-4278 Convert mongorestore_test.go index tests to testify #1082
• TOOLS-4278 Convert mongorestore options and basic restore tests to testify #1081
• TOOLS-4278 Convert mongorestore oplog tests to testify #1080
• TOOLS-4278 Convert mongorestore filepath tests to testify #1079
• TOOLS-4278 Convert mongorestore metadata and archive tests to testify #1078
• TOOLS-4278 Remove the last GoConvey from mongoimport #1077 👈 (View in Graphite)
• TOOLS-4278 Convert mongoimport TestImportDocuments to testify #1076
• TOOLS-4278 Convert mongoimport validation and reader tests to testify #1075
• TOOLS-4278 Convert mongoimport common tests to testify #1065
• TOOLS-4278 Convert mongoimport typed-fields tests to testify #1064
• TOOLS-4278 Convert mongoimport csv tests to testify #1063
• TOOLS-4278 Convert mongoimport tsv/json tests to testify #1062
• TOOLS-4278 Convert three smallest GoConvey test files to testify #1061
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[autarch]

This PR does slightly more than its title says, and the extra part is deliberate.

Alongside converting the last GoConvey in mongoimport, it consolidates three fixture-opening helpers into one openTestFixture in a new helpers_test.go. Earlier PRs in this stack each extracted their own while converting a single file, and the three came out functionally identical, differing only in name. Deduping them here rather than in those PRs keeps each of them a single-file diff, and this is the point where the whole package is converted and the duplication is finally visible.