Change struct keys from ion to strings for string tests

[jpschorr]

A handful of tests used quoted ion symbols as keys in structs, rather than strings. This seems to be a typo that happens to work as the ion is coerced into text.

These were originally ported from an old version of the PartiQL-Lang-Kotlin test scripts: https://github.com/partiql/partiql-lang-kotlin/blob/v0.7.0-alpha/testscript/pts/test-scripts/primitive-types/naughty-strings.sqlts#L20

This PR changes the keys to strings.

E.g.,

{ `key`: 'value' } -> { 'key': 'value'}

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[johnedquinn]

Approving, though it does make me think whether we even want to coerce symbols to strings in the future. I'm not entirely sure.

The intention of the original commit, though, seems clear that it was wrong to use Ion symbols for these tests.