make bevy math publishable

[mockersf]

Objective

• bevy_math fails to publish because of the self dev-dependency
• it's used to enable the approx feature in tests

Solution

• Don't specify a version in the dev-dependency. dependencies without a version are ignored by cargo when publishing
• Gate all the tests that depend on the approx feature so that it doesn't fail to compile when not enabled
• Also gate an import that wasn't used without bevy_reflect

Testing

• with at least cargo 1.84: cargo package -p bevy_math
• cd target/package/bevy_math_* && cargo test