CMakeLists.txt: Don't set MARL_BUILD_TESTS to OFF
CMakeLists.txt: Don't set MARL_BUILD_TESTS to OFF

This is now the default in marl.

Bug: b/140546382
Change-Id: Ic28537760b38c9ee051c8e527791495a57117466
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/36190
Tested-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>
Reviewed-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Kokoro-Presubmit: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader