spannertest: not failing as expected on queries with ambiguous column names

**Client**

spannertest

**Environment**

WSL Ubuntu

**Go Environment**

$ go version
go version go1.15.7 linux/amd64
$ go env

```
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/liza/.cache/go-build"
GOENV="/home/liza/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/liza/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/liza/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/bin/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/bin/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/d/git/src/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build043684261=/tmp/go-build -gno-record-gcc-switches"
```

**Code**

Rough example for purpose of this issue:

```ddl
CREATE TABLE NeighborhoodCats(
CatID INT64 NOT NULL,
CatName STRING(512) NOT NULL,
FavoriteFood String(512) NOT NULL,
) PRIMARY KEY(CatID);

CREATE TABLE HumanCatRelationships(
HumanID STRING(512) NOT NULL,
CatID INT64 NOT NULL,
) PRIMARY KEY(HumanID, CatID);
```

Query:

```sql
SELECT CatID, CatName, FavoriteFood
FROM HumanCatRelationships AS R INNER JOIN NeighborhoodCats AS C
ON R.CatID = C.CatID
WHERE HumanID = @humanID AND CatName = @catName
```

**Expected behavior**

I would expect the result of the above run against a live cloud spanner DB and against spannertest to be the same.

**Actual behavior**

spannertest runs the query above with no reported SQL errors whereas live spanner flags an ambiguous column:

`Column name CatID is ambiguous`

In this case this is a bit counterintuitive as CatID will have the same value as a result of the query, but the column name IS ambiguous and the query would be fixed with `SELECT R.CatID...`

Ideally, spannertest will fail on the same SQL errors as the live DB to ensure consistent behaviour during testing.