Flipping pref csp.speccompliant causes tests to fail on the B2G Emulator,
because test-container/index.html uses an inline-style which is blocked by CSP.
Rewrite of the *.html file to load the css/style information from an external file
CSP does not block it.
Merge pull request #18901 from ckerschb/cspTestContainerFix

Bug 858787 - Loading css for test-container from external file. r=fabrice