Spot market E2E tests are especially flaky

The spot market E2E tests make a real spot market bid, and are thus very sensitive to changes in spot market pricing. We've done some work in the past to stabilize them by making it more likely that the tests will pick a winning bid amount without exceeding the maximum bid, but we still sometimes fail to do that. From my vague memory of dealing with these tests before, if we bid lower the tests frequently (always?) time out because they didn't bid high enough to actually get a device.

One way to deal with this would be to set up the E2E tests to use a mock API for tests that need to win a device. If we don't already have tests that are meant to fail to get a device or meant to overbid, we should consider adding those; they can still hit the live API because we should be able to reliably under- or over-bid on purpose.

=== NAME  TestAccMetalSpotMarketRequestCreate_WithTimeout
    resource_metal_spot_market_request_acc_test.go:245: Step 1/1, expected an error with pattern, no match on: Error running apply: exit status 1

        Error: POST https://api.equinix.com/metal/v1/projects/0667bb71-9428-4919-8c1d-58fc0fb95613/spot-market-requests?include=devices%2Cproject%2Cplan: 422 2.21 exceeds the maximum bid price of 2.20 

          with equinix_metal_spot_market_request.request,
          on terraform_plugin_test.tf line 122, in resource "equinix_metal_spot_market_request" "request":
         122: resource "equinix_metal_spot_market_request" "request" {

Originally posted by @displague in https://github.com/equinix/terraform-provider-equinix/issues/501#issuecomment-1898723112

equinix / terraform-provider-equinix

Spot market E2E tests are especially flaky #525