[eslint-plugin] fix ts-modules-only-named invalid test cases

[jeremymeng]

There's some change in typescript-eslint that breaks some eslint test cases.
Previously when running tests, in a rule's create() method, we get a relative
path for context.filename, and when we pass a setting of { main: "src/test.ts" } in rule tester option, the two were considered the same file,
therefore the following condition check failed, and we continue to report error
for default exports.

azure-sdk-for-js/common/tools/eslint-plugin-azure-sdk/src/rules/ts-modules-only-named.ts

Line 31 in 6dd8b58

if (relative(normalize(context.filename), normalize(context.settings.main as string)) !== "") {

After the upgrade, context.filename now contains the full path, the file
src/test.ts in our test fixture has a full path of
/.../common/tools/eslint-plugin-azure-sdk/tests/fixture/src/test.ts. It is not
considered the same file as the passed settings implies
/.../common/tools/eslint-plugin-azure-sdk/src/test.ts when we run the tests.
So we return earlier instead of going to check for default exports.

This is not an issue in real scenario where we have settings of { main: "src/index.ts" }, which would match the full path of the file we are linting
when we run eslint from a package directory.

This PR fixes the test by passing tests/fixture/src/test.ts in the settings so
that it matches the full path of context.filename we get in create().